| Message ID | 20230714-drm-sched-fixes-v1-2-c567249709f7@asahilina.net |
|---|---|
| State | New |
| Headers | show |
| Series | [1/3] drm/scheduler: Add more documentation | expand |
Am 15.07.23 um 16:14 schrieb alyssa@rosenzweig.io: > 15 July 2023 at 00:03, "Luben Tuikov" <luben.tuikov@amd.com> wrote: >> On 2023-07-14 05:57, Christian König wrote: >> >>> Am 14.07.23 um 11:49 schrieb Asahi Lina: >>> >>>> On 14/07/2023 17.43, Christian König wrote: >>>> >>> Am 14.07.23 um 10:21 schrieb Asahi Lina: >>> A signaled scheduler fence can outlive its scheduler, since fences are >>> independencly reference counted. Therefore, we can't reference the >>> scheduler in the get_timeline_name() implementation. >>> >>> Fixes oopses on `cat /sys/kernel/debug/dma_buf/bufinfo` when shared >>> dma-bufs reference fences from GPU schedulers that no longer exist. >>> >>> Signed-off-by: Asahi Lina <lina@asahilina.net> >>> --- >>> drivers/gpu/drm/scheduler/sched_entity.c | 7 ++++++- >>> drivers/gpu/drm/scheduler/sched_fence.c | 4 +++- >>> include/drm/gpu_scheduler.h | 5 +++++ >>> 3 files changed, 14 insertions(+), 2 deletions(-) >>> >>> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c >>> b/drivers/gpu/drm/scheduler/sched_entity.c >>> index b2bbc8a68b30..17f35b0b005a 100644 >>> --- a/drivers/gpu/drm/scheduler/sched_entity.c >>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c >>> @@ -389,7 +389,12 @@ static bool >>> drm_sched_entity_add_dependency_cb(struct drm_sched_entity *entity) >>> /* >>> * Fence is from the same scheduler, only need to wait for >>> - * it to be scheduled >>> + * it to be scheduled. >>> + * >>> + * Note: s_fence->sched could have been freed and reallocated >>> + * as another scheduler. This false positive case is okay, >>> as if >>> + * the old scheduler was freed all of its jobs must have >>> + * signaled their completion fences. >>> >>> This is outright nonsense. As long as an entity for a scheduler exists >>> it is not allowed to free up this scheduler. >>> >>> So this function can't be called like this. >>> >>>> As I already explained, the fences can outlive their scheduler. That >>>> means *this* entity certainly exists for *this* scheduler, but the >>>> *dependency* fence might have come from a past scheduler that was >>>> already destroyed along with all of its entities, and its address reused. >>>> >>> >>> Well this is function is not about fences, this function is a callback >>> for the entity. >>> >>> >>>> Christian, I'm really getting tired of your tone. I don't appreciate >>>> being told my comments are "outright nonsense" when you don't even >>>> take the time to understand what the issue is and what I'm trying to >>>> do/document. If you aren't interested in working with me, I'm just >>>> going to give up on drm_sched, wait until Rust gets workqueue support, >>>> and reimplement it in Rust. You can keep your broken fence lifetime >>>> semantics and I'll do my own thing. >>>> >>> >>> I'm certainly trying to help here, but you seem to have unrealistic >>> expectations. >>> >>> I perfectly understand what you are trying to do, but you don't seem to >>> understand that this functionality here isn't made for your use case. >>> >>> We can adjust the functionality to better match your requirements, but >>> you can't say it is broken because it doesn't work when you use it not >>> in the way it is intended to be used. >>> >> I believe "adjusting" functionality to fit some external requirements, >> may have unintended consequences, requiring yet more and more "adjustments". >> (Or may allow (new) drivers to do wild things which may lead to wild results. :-) ) >> >> We need to be extra careful and wary of this. > Either drm/scheduler is common code that we should use for our driver, in which case we need to "adjust" it to fit the requirements of a safe Rust abstraction usable for AGX. Well this is the fundamental disagreement we have. As far as I can see you don't need to adjust anything in the common drm/scheduler code. That code works with quite a bunch of different drivers, including the Intel XE which has similar requirements to your work here. We can talk about gradually improving the common code, but as Luben already wrote as well this needs to be done very carefully. > Or, drm/scheduler is not common code intended for drivers with our requirements, and then we need to be able to write our own scheduler. > > AMD has NAK'd both options, effectively NAK'ing the driver. > > I will ask a simple yes/no question: Should we use drm/sched? Well, yes. > > If yes, it will need patches like these, No, you don't. First of all you need to try to adjust your driver to match the requirements of drm/scheduler and *not* the other way around. > and AMD needs to be ok with that and stop NAK'ing them on sight becuase they don't match the existing requirements. > > If no, we will write our own scheduler in Rust, and AMD needs to be ok with that and not NAK it on sight because it's not drm/sched. > > Which is it? > > Note if we write a Rust scheduler, drm/sched and amdgpu will be unaffected. If we do that and AMD comes back and NAKs it -- as said in this thread would "probably" happen -- then it is impossible for us to upstream a driver regardless of whether we use drm/sched. > > Lina has been polite and accommodating while AMD calls her code "outright nonsense" and gets "outright NAK"s, and puts her into an impossible catch-22 where no matter what she does it's NAK'd. Well as far as I can see I'm totally polite as well. Pointing out that approaches doesn't seem to make sense and NAKing patches is a perfectly normal part of the review process. What you need to to is to take a step back and ask yourself why this here is facing so much rejection from our side. I have to admit that I don't seem to be good at explaining that, cause we are obviously talking past each other, but you don't seem to try hard to understand what I'm pointing out either. > That's not ok. As far as I can see it is. As maintainer of a commonly used component my first duty is to preserve the status quo and prevent modifications which are not well thought through. And to be honest those changes here strongly looks like Lina is just adjusting the code to match her requirements without looking left and right first. Regards, Christian.
On 18/07/2023 00.55, Christian König wrote: > Am 15.07.23 um 16:14 schrieb alyssa@rosenzweig.io: >> 15 July 2023 at 00:03, "Luben Tuikov" <luben.tuikov@amd.com> wrote: >>> On 2023-07-14 05:57, Christian König wrote: >>> >>>> Am 14.07.23 um 11:49 schrieb Asahi Lina: >>>> >>>>> On 14/07/2023 17.43, Christian König wrote: >>>>> >>>> Am 14.07.23 um 10:21 schrieb Asahi Lina: >>>> A signaled scheduler fence can outlive its scheduler, since fences are >>>> independencly reference counted. Therefore, we can't reference the >>>> scheduler in the get_timeline_name() implementation. >>>> >>>> Fixes oopses on `cat /sys/kernel/debug/dma_buf/bufinfo` when shared >>>> dma-bufs reference fences from GPU schedulers that no longer exist. >>>> >>>> Signed-off-by: Asahi Lina <lina@asahilina.net> >>>> --- >>>> drivers/gpu/drm/scheduler/sched_entity.c | 7 ++++++- >>>> drivers/gpu/drm/scheduler/sched_fence.c | 4 +++- >>>> include/drm/gpu_scheduler.h | 5 +++++ >>>> 3 files changed, 14 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c >>>> b/drivers/gpu/drm/scheduler/sched_entity.c >>>> index b2bbc8a68b30..17f35b0b005a 100644 >>>> --- a/drivers/gpu/drm/scheduler/sched_entity.c >>>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c >>>> @@ -389,7 +389,12 @@ static bool >>>> drm_sched_entity_add_dependency_cb(struct drm_sched_entity *entity) >>>> /* >>>> * Fence is from the same scheduler, only need to wait for >>>> - * it to be scheduled >>>> + * it to be scheduled. >>>> + * >>>> + * Note: s_fence->sched could have been freed and reallocated >>>> + * as another scheduler. This false positive case is okay, >>>> as if >>>> + * the old scheduler was freed all of its jobs must have >>>> + * signaled their completion fences. >>>> >>>> This is outright nonsense. As long as an entity for a scheduler exists >>>> it is not allowed to free up this scheduler. >>>> >>>> So this function can't be called like this. >>>> >>>>> As I already explained, the fences can outlive their scheduler. That >>>>> means *this* entity certainly exists for *this* scheduler, but the >>>>> *dependency* fence might have come from a past scheduler that was >>>>> already destroyed along with all of its entities, and its address reused. >>>>> >>>> >>>> Well this is function is not about fences, this function is a callback >>>> for the entity. >>>> >>>> >>>>> Christian, I'm really getting tired of your tone. I don't appreciate >>>>> being told my comments are "outright nonsense" when you don't even >>>>> take the time to understand what the issue is and what I'm trying to >>>>> do/document. If you aren't interested in working with me, I'm just >>>>> going to give up on drm_sched, wait until Rust gets workqueue support, >>>>> and reimplement it in Rust. You can keep your broken fence lifetime >>>>> semantics and I'll do my own thing. >>>>> >>>> >>>> I'm certainly trying to help here, but you seem to have unrealistic >>>> expectations. >>>> >>>> I perfectly understand what you are trying to do, but you don't seem to >>>> understand that this functionality here isn't made for your use case. >>>> >>>> We can adjust the functionality to better match your requirements, but >>>> you can't say it is broken because it doesn't work when you use it not >>>> in the way it is intended to be used. >>>> >>> I believe "adjusting" functionality to fit some external requirements, >>> may have unintended consequences, requiring yet more and more "adjustments". >>> (Or may allow (new) drivers to do wild things which may lead to wild results. :-) ) >>> >>> We need to be extra careful and wary of this. >> Either drm/scheduler is common code that we should use for our driver, in which case we need to "adjust" it to fit the requirements of a safe Rust abstraction usable for AGX. > > Well this is the fundamental disagreement we have. As far as I can see > you don't need to adjust anything in the common drm/scheduler code. > > That code works with quite a bunch of different drivers, including the > Intel XE which has similar requirements to your work here. > > We can talk about gradually improving the common code, but as Luben > already wrote as well this needs to be done very carefully. > >> Or, drm/scheduler is not common code intended for drivers with our requirements, and then we need to be able to write our own scheduler. >> >> AMD has NAK'd both options, effectively NAK'ing the driver. >> >> I will ask a simple yes/no question: Should we use drm/sched? > > Well, yes. > >> >> If yes, it will need patches like these, > > No, you don't. > > First of all you need to try to adjust your driver to match the > requirements of drm/scheduler and *not* the other way around. > >> and AMD needs to be ok with that and stop NAK'ing them on sight becuase they don't match the existing requirements. >> >> If no, we will write our own scheduler in Rust, and AMD needs to be ok with that and not NAK it on sight because it's not drm/sched. >> >> Which is it? >> >> Note if we write a Rust scheduler, drm/sched and amdgpu will be unaffected. If we do that and AMD comes back and NAKs it -- as said in this thread would "probably" happen -- then it is impossible for us to upstream a driver regardless of whether we use drm/sched. >> >> Lina has been polite and accommodating while AMD calls her code "outright nonsense" and gets "outright NAK"s, and puts her into an impossible catch-22 where no matter what she does it's NAK'd. > > Well as far as I can see I'm totally polite as well. > > Pointing out that approaches doesn't seem to make sense and NAKing > patches is a perfectly normal part of the review process. > > What you need to to is to take a step back and ask yourself why this > here is facing so much rejection from our side. I have to admit that I > don't seem to be good at explaining that, cause we are obviously talking > past each other, but you don't seem to try hard to understand what I'm > pointing out either. > >> That's not ok. > > As far as I can see it is. > > As maintainer of a commonly used component my first duty is to preserve > the status quo and prevent modifications which are not well thought > through. And to be honest those changes here strongly looks like Lina is > just adjusting the code to match her requirements without looking left > and right first. > > Regards, > Christian. > > I give up. You are ignoring everything we say, and rejecting everything we suggest. We've already explained why drm_sched doesn't work for us. I'm tired of repeating the same explanation over and over again only to be ignored and told I'm wrong. I'll start working on a new, much simpler Rust-native scheduler based on the workqueue Rust abstractions which are in review. ~~ Lina
On 2023-07-17 22:35, Asahi Lina wrote: > On 18/07/2023 00.55, Christian König wrote: >> Am 15.07.23 um 16:14 schrieb alyssa@rosenzweig.io: >>> 15 July 2023 at 00:03, "Luben Tuikov" <luben.tuikov@amd.com> wrote: >>>> On 2023-07-14 05:57, Christian König wrote: >>>> >>>>> Am 14.07.23 um 11:49 schrieb Asahi Lina: >>>>> >>>>>> On 14/07/2023 17.43, Christian König wrote: >>>>>> >>>>> Am 14.07.23 um 10:21 schrieb Asahi Lina: >>>>> A signaled scheduler fence can outlive its scheduler, since fences are >>>>> independencly reference counted. Therefore, we can't reference the >>>>> scheduler in the get_timeline_name() implementation. >>>>> >>>>> Fixes oopses on `cat /sys/kernel/debug/dma_buf/bufinfo` when shared >>>>> dma-bufs reference fences from GPU schedulers that no longer exist. >>>>> >>>>> Signed-off-by: Asahi Lina <lina@asahilina.net> >>>>> --- >>>>> drivers/gpu/drm/scheduler/sched_entity.c | 7 ++++++- >>>>> drivers/gpu/drm/scheduler/sched_fence.c | 4 +++- >>>>> include/drm/gpu_scheduler.h | 5 +++++ >>>>> 3 files changed, 14 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c >>>>> b/drivers/gpu/drm/scheduler/sched_entity.c >>>>> index b2bbc8a68b30..17f35b0b005a 100644 >>>>> --- a/drivers/gpu/drm/scheduler/sched_entity.c >>>>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c >>>>> @@ -389,7 +389,12 @@ static bool >>>>> drm_sched_entity_add_dependency_cb(struct drm_sched_entity *entity) >>>>> /* >>>>> * Fence is from the same scheduler, only need to wait for >>>>> - * it to be scheduled >>>>> + * it to be scheduled. >>>>> + * >>>>> + * Note: s_fence->sched could have been freed and reallocated >>>>> + * as another scheduler. This false positive case is okay, >>>>> as if >>>>> + * the old scheduler was freed all of its jobs must have >>>>> + * signaled their completion fences. >>>>> >>>>> This is outright nonsense. As long as an entity for a scheduler exists >>>>> it is not allowed to free up this scheduler. >>>>> >>>>> So this function can't be called like this. >>>>> >>>>>> As I already explained, the fences can outlive their scheduler. That >>>>>> means *this* entity certainly exists for *this* scheduler, but the >>>>>> *dependency* fence might have come from a past scheduler that was >>>>>> already destroyed along with all of its entities, and its address reused. >>>>>> >>>>> >>>>> Well this is function is not about fences, this function is a callback >>>>> for the entity. >>>>> >>>>> >>>>>> Christian, I'm really getting tired of your tone. I don't appreciate >>>>>> being told my comments are "outright nonsense" when you don't even >>>>>> take the time to understand what the issue is and what I'm trying to >>>>>> do/document. If you aren't interested in working with me, I'm just >>>>>> going to give up on drm_sched, wait until Rust gets workqueue support, >>>>>> and reimplement it in Rust. You can keep your broken fence lifetime >>>>>> semantics and I'll do my own thing. >>>>>> >>>>> >>>>> I'm certainly trying to help here, but you seem to have unrealistic >>>>> expectations. >>>>> >>>>> I perfectly understand what you are trying to do, but you don't seem to >>>>> understand that this functionality here isn't made for your use case. >>>>> >>>>> We can adjust the functionality to better match your requirements, but >>>>> you can't say it is broken because it doesn't work when you use it not >>>>> in the way it is intended to be used. >>>>> >>>> I believe "adjusting" functionality to fit some external requirements, >>>> may have unintended consequences, requiring yet more and more "adjustments". >>>> (Or may allow (new) drivers to do wild things which may lead to wild results. :-) ) >>>> >>>> We need to be extra careful and wary of this. >>> Either drm/scheduler is common code that we should use for our driver, in which case we need to "adjust" it to fit the requirements of a safe Rust abstraction usable for AGX. >> >> Well this is the fundamental disagreement we have. As far as I can see >> you don't need to adjust anything in the common drm/scheduler code. >> >> That code works with quite a bunch of different drivers, including the >> Intel XE which has similar requirements to your work here. >> >> We can talk about gradually improving the common code, but as Luben >> already wrote as well this needs to be done very carefully. >> >>> Or, drm/scheduler is not common code intended for drivers with our requirements, and then we need to be able to write our own scheduler. >>> >>> AMD has NAK'd both options, effectively NAK'ing the driver. >>> >>> I will ask a simple yes/no question: Should we use drm/sched? >> >> Well, yes. >> >>> >>> If yes, it will need patches like these, >> >> No, you don't. >> >> First of all you need to try to adjust your driver to match the >> requirements of drm/scheduler and *not* the other way around. >> >>> and AMD needs to be ok with that and stop NAK'ing them on sight becuase they don't match the existing requirements. >>> >>> If no, we will write our own scheduler in Rust, and AMD needs to be ok with that and not NAK it on sight because it's not drm/sched. >>> >>> Which is it? >>> >>> Note if we write a Rust scheduler, drm/sched and amdgpu will be unaffected. If we do that and AMD comes back and NAKs it -- as said in this thread would "probably" happen -- then it is impossible for us to upstream a driver regardless of whether we use drm/sched. >>> >>> Lina has been polite and accommodating while AMD calls her code "outright nonsense" and gets "outright NAK"s, and puts her into an impossible catch-22 where no matter what she does it's NAK'd. >> >> Well as far as I can see I'm totally polite as well. >> >> Pointing out that approaches doesn't seem to make sense and NAKing >> patches is a perfectly normal part of the review process. >> >> What you need to to is to take a step back and ask yourself why this >> here is facing so much rejection from our side. I have to admit that I >> don't seem to be good at explaining that, cause we are obviously talking >> past each other, but you don't seem to try hard to understand what I'm >> pointing out either. >> >>> That's not ok. >> >> As far as I can see it is. >> >> As maintainer of a commonly used component my first duty is to preserve >> the status quo and prevent modifications which are not well thought >> through. And to be honest those changes here strongly looks like Lina is >> just adjusting the code to match her requirements without looking left >> and right first. >> >> Regards, >> Christian. >> >> > > I give up. You are ignoring everything we say, and rejecting everything > we suggest. We've already explained why drm_sched doesn't work for us. > I'm tired of repeating the same explanation over and over again only to > be ignored and told I'm wrong. > > I'll start working on a new, much simpler Rust-native scheduler based on > the workqueue Rust abstractions which are in review. > > ~~ Lina > Perhaps it is worth having a reset and just trying to clarify requirements one at a time, even if that involves describing a change on a single line in a single file. The maintainer discourse is quite common. Its ultimate goal is to keep things working. If we let some dependencies loose, or change some requirements, it's conceivable that this may lead to further problems with new development of current drivers, as well as new drivers. This will lead to more one-off fixes, and more "adjustments" to the point where the core requirement is lost, and the code has lost its purpose and meaning. Maintainers usually see 10 moves ahead, while driver developers as such in their role, are expressly concerned with their immediate need to get this and that driver and its feature working. We should perhaps concentrate on the core of the requirements--the very root of what is deemed to need to be changed, to understand why, and if there's a better way to achieve this, and in general a good reason for the way to proceed forward, whichever way is taken. Let's take it one thing at a time, slow and perhaps in a ELI5 manner to make sure no one misses the other person's point. Perhaps short and lucid, but complete emails would be best, with code quoted, and scenarios explained. I know this takes a long time--it's not unusual for me to take hours to write a single email and I'm exhausted after that. We all mean well here. Hope we can make something good.
On Mon, 17 Jul 2023 17:55:04 +0200 Christian König <christian.koenig@amd.com> wrote: > Am 15.07.23 um 16:14 schrieb alyssa@rosenzweig.io: ... > > Lina has been polite and accommodating while AMD calls her code > > "outright nonsense" and gets "outright NAK"s, and puts her into an > > impossible catch-22 where no matter what she does it's NAK'd. > > Well as far as I can see I'm totally polite as well. Christian, politeness is in the eye of the beholder. You do not get to decide how other people feel. I consider myself a very blunt and difficult reviewer on my own area (which I consider mostly as a negative trait), and while I have alienated some people over the years, I try hard to not intentionally hurt anyone. Sometimes it means that writing one email takes an hour or two. It can take a tremendous amount of energy. Like this email here. If people have the courage to repeatedly tell someone that the someone comes out as off-putting, it cannot be dismissed. It really means coming out as off-putting. There does not need to be anything malicious related to it from either side, it could as well be a cultural difference that one cannot know in advance, or it could be a personal hurt inside the offending person lashing out. When told, it is time to reflect. > Pointing out that approaches doesn't seem to make sense and NAKing > patches is a perfectly normal part of the review process. Yes. You don't have to change your message. One only needs to make an effort to try to change their tone. Otherwise they lose and alienate developers by choosing to hurt them. It was an accident before one knew about it, but now it is known, so how one communicates is a decision. It's no longer an accident. > What you need to to is to take a step back and ask yourself why this > here is facing so much rejection from our side. I have to admit that I > don't seem to be good at explaining that, cause we are obviously talking > past each other, but you don't seem to try hard to understand what I'm > pointing out either. Maybe try using a softer tone for a start? Lina has reiterated the restrictions imposed by the hardware, the firmware they cannot change, and Rust design principles. How do *you* fit those with unchanged drm/sched? > > That's not ok. > > As far as I can see it is. Hurting people is not ok. Not even if the kernel community culture traditionally does so. > As maintainer of a commonly used component my first duty is to preserve > the status quo and prevent modifications which are not well thought > through. Of course. Accidentally hurting someone is eventually unavoidable. Defending the communication style that hurt someone in order to keep on doing that just makes one look like a d... Thanks, pq
Am 18.07.23 um 04:35 schrieb Asahi Lina: > On 18/07/2023 00.55, Christian König wrote: >> [SNIP] > > I give up. You are ignoring everything we say, and rejecting > everything we suggest. We've already explained why drm_sched doesn't > work for us. I'm tired of repeating the same explanation over and over > again only to be ignored and told I'm wrong. I'm not telling you that you are wrong in any way. What I'm pointing out is that your solution won't work upstream and you need to take a step back and think about why this won't work. > > I'll start working on a new, much simpler Rust-native scheduler based > on the workqueue Rust abstractions which are in review. Please note that when you are implementing a dma_fence interface you also need my acknowledgement to get this upstream. Additional to that Dave and Daniel might have objections to this as well. Regards, Christian. > > ~~ Lina >
Am 21.07.23 um 12:33 schrieb Asahi Lina: > [SNIP] > I've already tried to explain the issue. The DRM scheduler design, as > it stands today, makes it impractical to write a safe Rust abstraction > for it. This is a fact. Christian has repeatedly NAKed my attempts at > changing it to make such a safe abstraction possible, and is clearly > opposed to the fundamental lifetime requirements change I am trying to > make here. Therefore, we are at an impasse. > > It's unfortunate, but given this situation, the DRM scheduler will not > be available to Rust DRM drivers. I thought the goal of the DRM > subsystem common code was to cater to multiple drivers and usage > approaches, with an emphasis on doing things "right" and avoiding > design issues that are common mistakes in driver design. Clearly, this > is not the case for all of DRM, at least not the DRM scheduler. > > In software engineering, complex lifetime requirements are an > anti-pattern, which is one reason why Rust draws a line between safe > and unsafe APIs. For a safe API, it is up to the API developer to > design it such that it cannot be misused in a way that causes memory > safety issues, and the only lifetime requirements it can impose are > those that can be expressed in the Rust type system and statically > checked at compile time. The DRM scheduler's complex chain of lifetime > requirements cannot, and wrapping it in enough glue to remove those > lifetime requirements would require about as much code as just > rewriting it, as well as add unacceptable duplication and overhead. > > In kernel Rust, we strive to only have safe APIs for components which > have no fundamental reason for being unsafe (things like direct memory > mapping and raw hardware access). The DRM scheduler does not fall into > one of those "inherently unsafe" categories, so it doesn't make sense > to provide a raw unsafe API for it. This is not completely correct. The DRM scheduler provides a dma_fence interface as wrapper around hardware dma_fence interfaces. This interface is made to allow core Linux memory management to query the progress of hardware operations. So you are working with something very low level here and have to follow restrictions which Rust can't enforce from the language because it simply can't know about that at compile time. > Doing so would just expose Rust code to the kind of subtle safety > implications that cause memory problems every day in C. If I were to > use drm_sched's unsafe API "as is" in my driver, it would *by far* be > the least auditable, most complex usage of unsafe code in the entire > driver, and I have no confidence that I would be able to get it right > and keep it right as the driver changes. > > I don't see a reason why this cannot be simply fixed in drm_sched, but > Christian disagrees, and has repeatedly (and strongly) NAKed my > attempts at doing so, and indeed NAKed the entire premise of the > change in lifetime requirements itself. So here we are. There is no > solution that will work for everyone that involves drm_sched. > > So I don't have a choice other than to just not use drm_sched and roll > my own. I will happily move that Rust implementation to common code if > another Rust driver comes along and wants to use it. I'm not sure if > that kind of thing can be easily made available to C drivers in > reverse, but I guess we'll cross that bridge when a C driver expresses > interest in using it. Well, to make it clear once more: Signaling a dma_fence from the destructor of a reference counted object is very problematic! This will be rejected no matter if you do that in C or in Rust. What we can do is to make it safe in the sense that you don't access freed up memory by using the scheduler fences even more as wrapper around the hardware fence as we do now. But this quite a change and requires a bit more than just hacking around drm_sched_fence_get_timeline_name(). > > So far it seems existing C drivers are happy with drm_sched's design > and complex lifetime requirements, even though they aren't even > documented. Perhaps they managed to reverse engineer them from the > source, or someone told the authors about it (certainly nobody told me > when I started using drm_sched). Or maybe a bunch of the drm_scheduler > users are actually broken and have memory safety issues in corner > cases. I don't know, though if I had to bet, I'd bet on the second > option. > > Actually, let's do a quick search and find out! > > panfrost_remove() -> panfrost_device_fini() -> panfrost_job_fini() -> > drm_sched_fini() > > There is a direct, synchronous path between device removal and > destroying the DRM scheduler. At no point does it wait for any jobs to > complete. That's what patch #3 in this series tries to fix. > > In fact, it doesn't even keep the entities alive! It calls > drm_dev_unregister() before everything else, but as the docs for the > DRM driver lifetimes clearly say (see, docs!), objects visible to > userspace must survive that and only be released from the release > callback. drm_sched entities are created/destroyed from > panfrost_job_open()/panfrost_job_close(), which are called from > panfrost_open() and panfrost_postclose(), which are the userspace file > open/close functions. > > That one I fix in the Rust abstraction already (that one's relatively > easy to fix), so it doesn't need a drm_sched patch from my point of > view, but it is yet another subtle, undocumented lifetime requirement > that is, evidently, impossible to know about and get right without > documentation. > > Meanwhile, panfrost_fence_ops has no remove() callback, which means > there is no reference path stopping device removal (and therefore > scheduler teardown) or even module unload while driver fences are > still alive. That leads to the UAF patch #2 in this series tries to fix. > > In other words, as far as I can tell, the panfrost driver gets > *everything* wrong when it comes to the DRM scheduler lifetime > requirements, and will crash and burn if the driver is unbound while > jobs are in flight, anyone has a file descriptor open at all, or even > if any shared buffer holding a DRM scheduler fence from it is bound to > the display controller at that time. Yeah, that is perfectly correct what you wrote. Daniel and I have gone back and forth multiple times how we should design this and we opted to not use direct pointers for the contexts because that allows for simpler driver implementations. The DRM scheduler doesn't document the lifetime requirements because it doesn't define the lifetime requirements. From the design the DRM scheduler is supposed to be an component wrapping around DMA fences. And those DMA fences have the necessary lifetime definition. Now DMA fences have their live cycles explained in the structure documentation, but it doesn't really talk much about the requirements for dma_fence_ops implementations. We should probably improve that. So yes, drivers need to keep the structures which might be accessed by userspace alive even after the underlying device is removed. But signaling dma_fences is completely independent from that. > > This is why this kind of design is not allowed in Rust. Well it is allowed, it's just not safe. Regards, Christian. > Because nobody gets it right. *Especially* not without docs. I > assumed, like the authors of the Panfrost driver clearly assumed, that > the DRM scheduler API would not have these crazy undocumented hidden > requirements. The only reason I found out the hard way is I happen to > create and destroy schedulers all the time, not just once globally, so > I would hit the bugs and dangling pointers much more often than > Panfrost users who, most likely, never unbind their devices. But we > both have the same problem. > > I think I've done all I can to explain the issues and try to fix them, > so the ball is in your court now. If you want to keep the current > design, that's fine, but Rust code will not be a drm_sched user in > that case. And the rest of the DRM folks in the C world will have to > contend with these issues and fix all the problems in the C drivers > (I'm sure panfrost isn't the only one, it's just literally the first > one I looked at). > > As for me, I'm happy to write a simple workqueue-based Rust scheduler > suitable for firmware-managed scheduler devices. Honestly, at this > point, I have very little faith left in my ability to fix all these > issues in drm_sched (I know there's at least one more lurking, I saw a > NULL deref but I wasn't able to reproduce it nor divine how it > possibly happened). That, combined with the hostility from the AMD > folks about my attempts to improve drm_sched even just a little bit, > makes that decision very easy. > > Farewell, DRM scheduler. It was nice trying to work with you, but > things just didn't work out. I won't be submitting a v2 to this series > myself. Please ping me if you fix all these fundamental design issues > with drm_sched and think I might actually be able to use it safely in > Rust one day. If the API design is solid and safe and the > implementation done in a way that inspires confidence at that time > maybe we can yank out my Rust solution when the time comes and switch > back to drm_sched. > > Just please don't expect me to do the work any more, I've done > everything I can and this now has to come from you, not me. I've spent > way more time understanding drm_sched, auditing its code, > understanding its design intent, trying to fix it, and getting yelled > at for it than it would take to write a new, clean, safe Rust > scheduler. I don't regret some of the time spent (some of the > implementation details of drm_sched have taught me useful things), but > I'm not prepared to spend any more, sorry. > > ~~ Lina >
Am 01.11.23 um 09:13 schrieb Daniel Vetter: > On Wed, 1 Nov 2023 at 07:59, Dave Airlie <airlied@gmail.com> wrote: >>> Well, to make it clear once more: Signaling a dma_fence from the >>> destructor of a reference counted object is very problematic! This will >>> be rejected no matter if you do that in C or in Rust. >>> >>> What we can do is to make it safe in the sense that you don't access >>> freed up memory by using the scheduler fences even more as wrapper >>> around the hardware fence as we do now. But this quite a change and >>> requires a bit more than just hacking around >>> drm_sched_fence_get_timeline_name(). >> I really think this needs to be documented if nothing else out of this thread. >> >> Clearly nobody is going to get it right and hidden here in this >> thread, this info isn't useful. >> >> Can we have some sort of design document for the dma-fence/scheduler >> interactions written and we can try and refine it with solutions on >> the list, because I'm tired of people proposing things and NAK's >> getting thrown around without anything to point people at. >> >> The next NAK I see on the list will mean I block all patches from the >> sender until they write a documentation patch, because seriously this >> stuff is too hard for someone to just keep it in their head and expect >> everyone else to understand from reading the code. > I very much like the idea that NAK replies are counted as "you've just > volunteered yourself for some documentation patches so that next time > around you can reply with a link to the docs instead of just a NAK". Yeah, that sounds like a great idea to me as well :) Especially when I can use it to convince managers that we need to have more work force on writing documentation. > I don't think we'll get out of these discussions otherwise, since > currently we have undocumented, but very tricky semantics of the > drm/sched codebase for ringbuffer scheduling which is extended to fw > scheduling in also very tricky ways, with not entirely clear impacts > on semantics of all the drm/sched things. And as a result we just pile > up enormous amounts of threads where I think the only thing assured is > that people talk past each another. The scheduler is certainly the ugliest part, but it's unfortunately still only the tip of the iceberg. I have seen at least halve a dozen approach in the last two years where people tried to signal a dma_fence from userspace or similar. Fortunately it was mostly prototyping and I could jump in early enough to stop that, but basically this is a fight against windmills. I was considering to change the dma_fence semantics so that dma_fence_signal() could only be called from the interrupt contexts of devices and then put a big fat WARN_ON(!in_interrupt()) in there. It's a sledgehammer, but as far as I can see the only thing which might help. Opinions? Thanks, Christian. > > Converting NAKs into doc patches should at least eventually get rid of > the worst confusions we're dealing with here. > > Cheers, Sima
Am Donnerstag, dem 02.11.2023 um 11:48 +0100 schrieb Christian König: [...] > I was considering to change the dma_fence semantics so that > dma_fence_signal() could only be called from the interrupt contexts of > devices and then put a big fat WARN_ON(!in_interrupt()) in there. > > It's a sledgehammer, but as far as I can see the only thing which might > help. Opinions? That's not going to fly. As soon as you are dealing with device drivers that use IRQ threads, either voluntarily or even involuntarily on RT kernels, the dma_fence_signal will be from process context. Regards, Lucas
Am 02.11.23 um 12:19 schrieb Lucas Stach: > Am Donnerstag, dem 02.11.2023 um 11:48 +0100 schrieb Christian König: > [...] >> I was considering to change the dma_fence semantics so that >> dma_fence_signal() could only be called from the interrupt contexts of >> devices and then put a big fat WARN_ON(!in_interrupt()) in there. >> >> It's a sledgehammer, but as far as I can see the only thing which might >> help. Opinions? > That's not going to fly. As soon as you are dealing with device drivers > that use IRQ threads, either voluntarily or even involuntarily on RT > kernels, the dma_fence_signal will be from process context. Ah shit, yeah of course. We use IRQ threads in amdgpu for the second interrupt ring as well. Ok, nail that coffin. Any other ideas how we could enforce this? Thanks, Christian. > > Regards, > Lucas
diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c index b2bbc8a68b30..17f35b0b005a 100644 --- a/drivers/gpu/drm/scheduler/sched_entity.c +++ b/drivers/gpu/drm/scheduler/sched_entity.c @@ -389,7 +389,12 @@ static bool drm_sched_entity_add_dependency_cb(struct drm_sched_entity *entity) /* * Fence is from the same scheduler, only need to wait for - * it to be scheduled + * it to be scheduled. + * + * Note: s_fence->sched could have been freed and reallocated + * as another scheduler. This false positive case is okay, as if + * the old scheduler was freed all of its jobs must have + * signaled their completion fences. */ fence = dma_fence_get(&s_fence->scheduled); dma_fence_put(entity->dependency); diff --git a/drivers/gpu/drm/scheduler/sched_fence.c b/drivers/gpu/drm/scheduler/sched_fence.c index ef120475e7c6..06a0eebcca10 100644 --- a/drivers/gpu/drm/scheduler/sched_fence.c +++ b/drivers/gpu/drm/scheduler/sched_fence.c @@ -68,7 +68,7 @@ static const char *drm_sched_fence_get_driver_name(struct dma_fence *fence) static const char *drm_sched_fence_get_timeline_name(struct dma_fence *f) { struct drm_sched_fence *fence = to_drm_sched_fence(f); - return (const char *)fence->sched->name; + return (const char *)fence->sched_name; } static void drm_sched_fence_free_rcu(struct rcu_head *rcu) @@ -216,6 +216,8 @@ void drm_sched_fence_init(struct drm_sched_fence *fence, unsigned seq; fence->sched = entity->rq->sched; + strlcpy(fence->sched_name, entity->rq->sched->name, + sizeof(fence->sched_name)); seq = atomic_inc_return(&entity->fence_seq); dma_fence_init(&fence->scheduled, &drm_sched_fence_ops_scheduled, &fence->lock, entity->fence_context, seq); diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h index e95b4837e5a3..4fa9523bd47d 100644 --- a/include/drm/gpu_scheduler.h +++ b/include/drm/gpu_scheduler.h @@ -305,6 +305,11 @@ struct drm_sched_fence { * @lock: the lock used by the scheduled and the finished fences. */ spinlock_t lock; + /** + * @sched_name: the name of the scheduler that owns this fence. We + * keep a copy here since fences can outlive their scheduler. + */ + char sched_name[16]; /** * @owner: job owner for debugging */
A signaled scheduler fence can outlive its scheduler, since fences are independencly reference counted. Therefore, we can't reference the scheduler in the get_timeline_name() implementation. Fixes oopses on `cat /sys/kernel/debug/dma_buf/bufinfo` when shared dma-bufs reference fences from GPU schedulers that no longer exist. Signed-off-by: Asahi Lina <lina@asahilina.net> --- drivers/gpu/drm/scheduler/sched_entity.c | 7 ++++++- drivers/gpu/drm/scheduler/sched_fence.c | 4 +++- include/drm/gpu_scheduler.h | 5 +++++ 3 files changed, 14 insertions(+), 2 deletions(-)