| Message ID | 20170918204855.170920-1-arnd@arndb.de |
|---|---|
| State | New |
| Headers | show |
| Series | tcp: avoid bogus warning in tcp_clean_rtx_queue | expand |
From: Arnd Bergmann <arnd@arndb.de> Date: Mon, 18 Sep 2017 22:48:47 +0200 > gcc-4.9 warns that it cannot trace the state of the 'last_ackt' > variable since the change to the TCP timestamping code, when > CONFIG_PROFILE_ANNOTATED_BRANCHES is set: > > net/ipv4/tcp_input.c: In function 'tcp_clean_rtx_queue': > include/net/tcp.h:757:23: error: 'last_ackt' may be used uninitialized in this function [-Werror=maybe-uninitialized] > > Other gcc versions, both older and newer do now show this > warning. Removing the 'likely' annotation makes it go away, > and has no effect on the object code without > CONFIG_PROFILE_ANNOTATED_BRANCHES, as tested with gcc-4.9 > and gcc-7.1.1, so this seems to be a safe workaround. > > Fixes: 9a568de4818d ("tcp: switch TCP TS option (RFC 7323) to 1ms clock") > Signed-off-by: Arnd Bergmann <arnd@arndb.de> This reaches the limits at which I am willing to work around compiler stuff. What cpu did you test the object code generation upon and does that cpu have branch prediction hints in the target you are building for?
On Tue, Sep 19, 2017 at 11:02 PM, David Miller <davem@davemloft.net> wrote: > From: Arnd Bergmann <arnd@arndb.de> > Date: Mon, 18 Sep 2017 22:48:47 +0200 > >> gcc-4.9 warns that it cannot trace the state of the 'last_ackt' >> variable since the change to the TCP timestamping code, when >> CONFIG_PROFILE_ANNOTATED_BRANCHES is set: >> >> net/ipv4/tcp_input.c: In function 'tcp_clean_rtx_queue': >> include/net/tcp.h:757:23: error: 'last_ackt' may be used uninitialized in this function [-Werror=maybe-uninitialized] >> >> Other gcc versions, both older and newer do now show this >> warning. Removing the 'likely' annotation makes it go away, >> and has no effect on the object code without >> CONFIG_PROFILE_ANNOTATED_BRANCHES, as tested with gcc-4.9 >> and gcc-7.1.1, so this seems to be a safe workaround. >> >> Fixes: 9a568de4818d ("tcp: switch TCP TS option (RFC 7323) to 1ms clock") >> Signed-off-by: Arnd Bergmann <arnd@arndb.de> > > This reaches the limits at which I am willing to work around compiler > stuff. I see. It is a definitely a really obscure case, so if there is any doubt that the workaround is harmless, then we shouldn't take it. The warning only shows up on gcc-4.9 but not anything newer, and we disable -Wmaybe-uninitialized on all older versions because of the false positives. It's also possible that it needed a combination of multiple other options, not just CONFIG_PROFILE_ANNOTATED_BRANCHES. I build-tested with gcc-4.9 to see if anything would show up that we don't also get a warning for in gcc-7, and this came up once in several hundred randconfig builds across multiple architectures (no other new warnings appeared with gcc-4.9). > What cpu did you test the object code generation upon and does that > cpu have branch prediction hints in the target you are building for? This was a randconfig build targetting ARMv5. I'm pretty sure that has no such hint instructions. Arnd
From: Arnd Bergmann <arnd@arndb.de> Date: Tue, 19 Sep 2017 23:32:33 +0200 > On Tue, Sep 19, 2017 at 11:02 PM, David Miller <davem@davemloft.net> wrote: >> What cpu did you test the object code generation upon and does that >> cpu have branch prediction hints in the target you are building for? > > This was a randconfig build targetting ARMv5. I'm pretty sure that has > no such hint instructions. I just tested on sparc64 and it changed the branch prediction: .L2157: - brz,pn %i3, .L1898 ! first_ackt, + brz,pt %i2, .L1898 ! first_ackt, mov -1, %o2 !, seq_rtt_us
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index c5d7656beeee..c52bc8e35d4d 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -3173,7 +3173,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, if (skb && (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) flag |= FLAG_SACK_RENEGING; - if (likely(first_ackt) && !(flag & FLAG_RETRANS_DATA_ACKED)) { + if (first_ackt && !(flag & FLAG_RETRANS_DATA_ACKED)) { seq_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, first_ackt); ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, last_ackt); }
gcc-4.9 warns that it cannot trace the state of the 'last_ackt' variable since the change to the TCP timestamping code, when CONFIG_PROFILE_ANNOTATED_BRANCHES is set: net/ipv4/tcp_input.c: In function 'tcp_clean_rtx_queue': include/net/tcp.h:757:23: error: 'last_ackt' may be used uninitialized in this function [-Werror=maybe-uninitialized] Other gcc versions, both older and newer do now show this warning. Removing the 'likely' annotation makes it go away, and has no effect on the object code without CONFIG_PROFILE_ANNOTATED_BRANCHES, as tested with gcc-4.9 and gcc-7.1.1, so this seems to be a safe workaround. Fixes: 9a568de4818d ("tcp: switch TCP TS option (RFC 7323) to 1ms clock") Signed-off-by: Arnd Bergmann <arnd@arndb.de> --- net/ipv4/tcp_input.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) -- 2.9.0