PR/59132 CVS commit: src/sys/kern

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

The following reply was made to PR kern/59132; it has been noted by GNATS.

From: "Taylor R Campbell" <riastradh%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/59132 CVS commit: src/sys/kern
Date: Wed, 5 Mar 2025 14:01:55 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Wed Mar  5 14:01:55 UTC 2025
 
 Modified Files:
 	src/sys/kern: sys_futex.c
 
 Log Message:
 futex(2): Avoid returning early on timeout.
 
 Rounding in the arithmetic leading into cv_timedwait_sig, and any
 skew between the timecounter used by clock_gettime and the hardclock
 timer used to wake cv_timedwait_sig, can lead cv_timedwait_sig to
 wake up before the deadline as observable by clock_gettime.
 
 futex(FUTEX_WAIT) is not supposed to do that, so ignore when
 cv_timedwait_sig returns EWOULDBLOCK -- we'll notice the deadline has
 passed in the next iteration anyway, if it has actually passed.
 
 While here, make sure that we never pass less than 1 tick to
 cv_timedwait_sig -- that turns it into cv_wait_sig, to wait
 indefinitely with no timeout.
 
 With this change, I have not seen any failures as reported in:
 
 PR kern/59132: t_futex_ops:futex_wait_timeout_* sometimes fails on
 early wakeup
 
 Some instrumentation in futex_wait to count when cv_timedwait_sig
 returns early as measured by clock_gettime (not committed in this
 change, just local experiments) supports this hypothesis for the
 symptoms observed in the PR.
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.25 -r1.26 src/sys/kern/sys_futex.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.