Date: Tue, 15 Sep 2009 19:14:25 +0000 (UTC) From: Attilio Rao <attilio@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-8@freebsd.org Subject: svn commit: r197226 - stable/8/sys/kern Message-ID: <200909151914.n8FJEPGw000116@svn.freebsd.org>
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Author: attilio Date: Tue Sep 15 19:14:25 2009 New Revision: 197226 URL: http://svn.freebsd.org/changeset/base/197226 Log: MFC r197223: Fix sched_switch_migrate() by assuming locks cannot be shared and a deadlock between 3 different threads by acquiring both runqueue locks when doing the migration. Please note that this is a special condition as we want this fix in before RC1 as we assume it is critical and so it has been handled as an instant-merge. For the STABLE_7 branch, 1 week before the MFC is assumed. Approved by: re (kib) Modified: stable/8/sys/kern/sched_ule.c Modified: stable/8/sys/kern/sched_ule.c ============================================================================== --- stable/8/sys/kern/sched_ule.c Tue Sep 15 18:39:27 2009 (r197225) +++ stable/8/sys/kern/sched_ule.c Tue Sep 15 19:14:25 2009 (r197226) @@ -1749,19 +1749,19 @@ sched_switch_migrate(struct tdq *tdq, st */ spinlock_enter(); thread_block_switch(td); /* This releases the lock on tdq. */ - TDQ_LOCK(tdn); - tdq_add(tdn, td, flags); - tdq_notify(tdn, td); + /* - * After we unlock tdn the new cpu still can't switch into this - * thread until we've unblocked it in cpu_switch(). The lock - * pointers may match in the case of HTT cores. Don't unlock here - * or we can deadlock when the other CPU runs the IPI handler. + * Acquire both run-queue locks before placing the thread on the new + * run-queue to avoid deadlocks created by placing a thread with a + * blocked lock on the run-queue of a remote processor. The deadlock + * occurs when a third processor attempts to lock the two queues in + * question while the target processor is spinning with its own + * run-queue lock held while waiting for the blocked lock to clear. */ - if (TDQ_LOCKPTR(tdn) != TDQ_LOCKPTR(tdq)) { - TDQ_UNLOCK(tdn); - TDQ_LOCK(tdq); - } + tdq_lock_pair(tdn, tdq); + tdq_add(tdn, td, flags); + tdq_notify(tdn, td); + TDQ_UNLOCK(tdn); spinlock_exit(); #endif return (TDQ_LOCKPTR(tdn));
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