[BUG] I think sleepqueue need to be protected in sleepq_broadcast

John Baldwin jhb at freebsd.org
Tue Sep 2 20:09:32 UTC 2008

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 
> 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 
> 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 
> another wait channel, sleepq_broadcast() might incorrectly determine that 
> 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) {
 		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 
> regards to thread_lock() and turnstiles.
> The caller of thread_lock() might have performed sufficient locking to 
> 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

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