Date: Tue, 2 Sep 2008 16:08:56 -0400 From: John Baldwin <jhb@freebsd.org> To: Tor Egge <Tor.Egge@cvsup.no.freebsd.org> Cc: attilio@freebsd.org, kevinxlinuz@163.com, freebsd-current@freebsd.org, Benjamin.Close@clearchain.com, kib@freebsd.org Subject: Re: [BUG] I think sleepqueue need to be protected in sleepq_broadcast Message-ID: <200809021608.57542.jhb@freebsd.org> In-Reply-To: <20080901.013117.74700691.Tor.Egge@cvsup.no.freebsd.org> References: <200808230003.44081.jhb@freebsd.org> <48B6BC81.5060300@clearchain.com> <20080901.013117.74700691.Tor.Egge@cvsup.no.freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sunday 31 August 2008 09:31:17 pm Tor Egge wrote:
>
> sleepq_resume_thread() contains an ownership handover of sq if the resumed
> thread is the last one blocked on the wait channel. After the handover, sq
is
> no longer protected by the sleep queue chain lock and should no longer be
> accessed by sleepq_broadcast().
>
> Normally, when sleepq_broadcast() incorrectly accesses sq after the
handover,
> it will find the sq->sq_blocked queue to be empty, and the code appears to
> work.
>
> If the last correctly woken thread manages to go to sleep again very quickly
on
> another wait channel, sleepq_broadcast() might incorrectly determine that
the
> sq->sq_blocked queue isn't empty, and start doing the wrong thing.
So disregard my earlier e-mail. Here is a simple fix for the sleepq case:
Index: subr_sleepqueue.c
===================================================================
--- subr_sleepqueue.c (revision 182679)
+++ subr_sleepqueue.c (working copy)
@@ -779,7 +779,7 @@
sleepq_broadcast(void *wchan, int flags, int pri, int queue)
{
struct sleepqueue *sq;
- struct thread *td;
+ struct thread *td, *tdn;
int wakeup_swapper;
CTR2(KTR_PROC, "sleepq_broadcast(%p, %d)", wchan, flags);
@@ -793,8 +793,7 @@
/* Resume all blocked threads on the sleep queue. */
wakeup_swapper = 0;
- while (!TAILQ_EMPTY(&sq->sq_blocked[queue])) {
- td = TAILQ_FIRST(&sq->sq_blocked[queue]);
+ TAILQ_FOREACH_SAFE(td, &sq->sq_blocked[queue], td_slpq, tdn) {
thread_lock(td);
if (sleepq_resume_thread(sq, td, pri))
wakeup_swapper = 1;
This only uses 'sq' to fetch the head of the queue once up front. It won't
use it again once it has started waking up threads.
> A similar (but probably much more difficult to trigger) issue is present
with
> regards to thread_lock() and turnstiles.
>
> The caller of thread_lock() might have performed sufficient locking to
ensure
> that the thread to be locked doesn't go away, but any turnstile spin lock
> pointed to by td->td_lock isn't protected. Making turnstiles type stable
> (setting UMA_ZONE_NOFREE flag for turnstile_zone) should fix that issue.
Note that unlike the sleepq case, turnstiles are not made runnable until all
of them are dequeued from the turnstile and assigned a new turnstile. Only
after all that is settled are the threads made runnable in turnstile_unpend().
However, that doesn't fix this specific race (though it means the turnstile
code is not subject to the same exact race as the sleepq code above). Making
turnstiles type-stable is indeed probably the only fix for this. :-/
--
John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200809021608.57542.jhb>