Date: Thu, 24 Sep 2009 09:08:23 +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-7@freebsd.org Subject: svn commit: r197453 - in stable/7/sys: . contrib/pf kern Message-ID: <200909240908.n8O98Ncg091256@svn.freebsd.org>
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Author: attilio Date: Thu Sep 24 09:08:22 2009 New Revision: 197453 URL: http://svn.freebsd.org/changeset/base/197453 Log: MFC r197223: Fix a deadlock in sched_switch_migrate given by the runqueues being not locked at once together when doing the switch. Modified: stable/7/sys/ (props changed) stable/7/sys/contrib/pf/ (props changed) stable/7/sys/kern/sched_ule.c Modified: stable/7/sys/kern/sched_ule.c ============================================================================== --- stable/7/sys/kern/sched_ule.c Thu Sep 24 08:35:17 2009 (r197452) +++ stable/7/sys/kern/sched_ule.c Thu Sep 24 09:08:22 2009 (r197453) @@ -1822,18 +1822,24 @@ 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(td->td_sched); - /* - * 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); + if (TDQ_LOCKPTR(tdn) == TDQ_LOCKPTR(tdq)) { TDQ_LOCK(tdq); + tdq_add(tdn, td, flags); + tdq_notify(td->td_sched); + } else { + tdq_lock_pair(tdn, tdq); + tdq_add(tdn, td, flags); + tdq_notify(td->td_sched); + TDQ_UNLOCK(tdn); } spinlock_exit(); #endif
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