2008/9/2, John Baldwin <jhb_at_freebsd.org>: > 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) > _at__at_ -779,7 +779,7 _at__at_ > > 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); > > _at__at_ -793,8 +793,7 _at__at_ > > > /* 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. :-/ What about setting a flag meaning "turnstile recycled but still used" and allow turnstile_wait() to spin until it gets unset before to access to td_turnstile? Thanks, Attilio -- Peace can only be achieved by understanding - A. EinsteinReceived on Thu Sep 04 2008 - 21:38:39 UTC
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