CVS commit: src/sys

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

Module Name:    src
Committed By:   riastradh
Date:           Wed Jun 29 22:27:01 UTC 2022

Modified Files:
        src/sys/kern: kern_condvar.c kern_sleepq.c kern_synch.c kern_timeout.c
            kern_turnstile.c sys_lwp.c sys_select.c
        src/sys/sys: sleepq.h

Log Message:
sleepq(9): Pass syncobj through to sleepq_block.

Previously the usage pattern was:

sleepq_enter(sq, l, lock);              // locks l
...
sleepq_enqueue(sq, ..., sobj, ...);     // assumes l locked, sets l_syncobj
... (*)
sleepq_block(...);                      // unlocks l

As long as l remains locked from sleepq_enter to sleepq_block,
l_syncobj is stable, and sleepq_block uses it via ktrcsw to determine
whether the sleep is on a mutex in order to avoid creating ktrace
context-switch records (which involves allocation which is forbidden
in softint context, while taking and even sleeping for a mutex is
allowed).

However, in turnstile_block, the logic at (*) also involves
turnstile_lendpri, which sometimes unlocks and relocks l.  At that
point, another thread can swoop in and sleepq_remove l, which sets
l_syncobj to sched_syncobj.  If that happens, ktrcsw does what is
forbidden -- tries to allocate a ktrace record for the context
switch.

As an optimization, sleepq_block or turnstile_block could stop early
if it detects that l_syncobj doesn't match -- we've already been
requested to wake up at this point so there's no need to mi_switch.
(And then it would be unnecessary to pass the syncobj through
sleepq_block, because l_syncobj would remain stable.)  But I'll leave
that to another change.

Reported-by: syzbot+8b9d7b066c32dbcdc63b%syzkaller.appspotmail.com@localhost


To generate a diff of this commit:
cvs rdiff -u -r1.53 -r1.54 src/sys/kern/kern_condvar.c
cvs rdiff -u -r1.72 -r1.73 src/sys/kern/kern_sleepq.c
cvs rdiff -u -r1.350 -r1.351 src/sys/kern/kern_synch.c
cvs rdiff -u -r1.69 -r1.70 src/sys/kern/kern_timeout.c
cvs rdiff -u -r1.43 -r1.44 src/sys/kern/kern_turnstile.c
cvs rdiff -u -r1.82 -r1.83 src/sys/kern/sys_lwp.c
cvs rdiff -u -r1.59 -r1.60 src/sys/kern/sys_select.c
cvs rdiff -u -r1.34 -r1.35 src/sys/sys/sleepq.h

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.