[v2] scsi: ufs: core: fix devfreq deadlocks

There is a lock inversion and rwsem read-lock recursion in the devfreq
target callback which can lead to deadlocks.

Specifically, ufshcd_devfreq_scale() already holds a clk_scaling_lock
read lock when toggling the write booster, which involves taking the
dev_cmd mutex before taking another clk_scaling_lock read lock.

This can lead to a deadlock if another thread:

  1) tries to acquire the dev_cmd and clk_scaling locks in the correct
     order, or

  2) takes a clk_scaling write lock before the attempt to take the
     clk_scaling read lock a second time.

Fix this by dropping the clk_scaling_lock before toggling the write
booster as was done before commit 0e9d4ca43ba8 ("scsi: ufs: Protect some
contexts from unexpected clock scaling").

While the devfreq callbacks are already serialised, add a second
serialising mutex to handle the unlikely case where a callback triggered
through the devfreq sysfs interface is racing with a request to disable
clock scaling through the UFS controller 'clkscale_enable' sysfs
attribute. This could otherwise lead to the write booster being left
disabled after having disabled clock scaling.

Also take the new mutex in ufshcd_clk_scaling_allow() to make sure that
any pending write booster update has completed on return.

Note that this currently only affects Qualcomm platforms since commit
87bd05016a64 ("scsi: ufs: core: Allow host driver to disable wb toggling
during clock scaling").

The lock inversion (i.e. 1 above) was reported by lockdep as:

 ======================================================
 WARNING: possible circular locking dependency detected
 6.1.0-next-20221216 #211 Not tainted
 ------------------------------------------------------
 kworker/u16:2/71 is trying to acquire lock:
 ffff076280ba98a0 (&hba->dev_cmd.lock){+.+.}-{3:3}, at: ufshcd_query_flag+0x50/0x1c0

 but task is already holding lock:
 ffff076280ba9cf0 (&hba->clk_scaling_lock){++++}-{3:3}, at: ufshcd_devfreq_scale+0x2b8/0x380

 which lock already depends on the new lock.
[  +0.011606]
 the existing dependency chain (in reverse order) is:

 -> #1 (&hba->clk_scaling_lock){++++}-{3:3}:
        lock_acquire+0x68/0x90
        down_read+0x58/0x80
        ufshcd_exec_dev_cmd+0x70/0x2c0
        ufshcd_verify_dev_init+0x68/0x170
        ufshcd_probe_hba+0x398/0x1180
        ufshcd_async_scan+0x30/0x320
        async_run_entry_fn+0x34/0x150
        process_one_work+0x288/0x6c0
        worker_thread+0x74/0x450
        kthread+0x118/0x120
        ret_from_fork+0x10/0x20

 -> #0 (&hba->dev_cmd.lock){+.+.}-{3:3}:
        __lock_acquire+0x12a0/0x2240
        lock_acquire.part.0+0xcc/0x220
        lock_acquire+0x68/0x90
        __mutex_lock+0x98/0x430
        mutex_lock_nested+0x2c/0x40
        ufshcd_query_flag+0x50/0x1c0
        ufshcd_query_flag_retry+0x64/0x100
        ufshcd_wb_toggle+0x5c/0x120
        ufshcd_devfreq_scale+0x2c4/0x380
        ufshcd_devfreq_target+0xf4/0x230
        devfreq_set_target+0x84/0x2f0
        devfreq_update_target+0xc4/0xf0
        devfreq_monitor+0x38/0x1f0
        process_one_work+0x288/0x6c0
        worker_thread+0x74/0x450
        kthread+0x118/0x120
        ret_from_fork+0x10/0x20

 other info that might help us debug this:
  Possible unsafe locking scenario:
        CPU0                    CPU1
        ----                    ----
   lock(&hba->clk_scaling_lock);
                                lock(&hba->dev_cmd.lock);
                                lock(&hba->clk_scaling_lock);
   lock(&hba->dev_cmd.lock);

  *** DEADLOCK ***

Fixes: 0e9d4ca43ba8 ("scsi: ufs: Protect some contexts from unexpected clock scaling")
Cc: stable@vger.kernel.org      # 5.12
Cc: Can Guo <quic_cang@quicinc.com>
Tested-by: Andrew Halaney <ahalaney@redhat.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
---

This issue has apparently been known for over a year [1] without anyone
bothering to fix it. Aside from the potential deadlocks, this also leads
to developers using Qualcomm platforms not being able to use lockdep to
prevent further issues like this from being introduced in other places.

Johan

[1] https://lore.kernel.org/lkml/1631843521-2863-1-git-send-email-cang@codeaurora.org


Changes in v2
 - leave the write booster unchanged on errors (Bart)


 drivers/ufs/core/ufshcd.c | 29 +++++++++++++++--------------
 include/ufs/ufshcd.h      |  2 ++
 2 files changed, 17 insertions(+), 14 deletions(-)

On 1/16/23 08:12, Johan Hovold wrote:
> There is a lock inversion and rwsem read-lock recursion in the devfreq
> target callback which can lead to deadlocks.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>

On Mon, 16 Jan 2023 17:12:01 +0100, Johan Hovold wrote:

> There is a lock inversion and rwsem read-lock recursion in the devfreq
> target callback which can lead to deadlocks.
> 
> Specifically, ufshcd_devfreq_scale() already holds a clk_scaling_lock
> read lock when toggling the write booster, which involves taking the
> dev_cmd mutex before taking another clk_scaling_lock read lock.
> 
> [...]

Applied to 6.2/scsi-fixes, thanks!

[1/1] scsi: ufs: core: fix devfreq deadlocks
      https://git.kernel.org/mkp/scsi/c/ba81043753ff