From 9cede8fb41b6a41a7a10d5908dc2030b7d840b5f Mon Sep 17 00:00:00 2001 From: Attilio Rao Date: Tue, 15 Sep 2009 19:14:25 +0000 Subject: [PATCH] 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) --- sys/kern/sched_ule.c | 22 +++++++++++----------- 1 file changed, 11 insertions(+), 11 deletions(-) diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c index 27e8371dff2..fd14fc4021a 100644 --- a/sys/kern/sched_ule.c +++ b/sys/kern/sched_ule.c @@ -1749,19 +1749,19 @@ sched_switch_migrate(struct tdq *tdq, struct thread *td, int flags) */ spinlock_enter(); thread_block_switch(td); /* This releases the lock on tdq. */ - TDQ_LOCK(tdn); + + /* + * 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. + */ + tdq_lock_pair(tdn, tdq); 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. - */ - if (TDQ_LOCKPTR(tdn) != TDQ_LOCKPTR(tdq)) { - TDQ_UNLOCK(tdn); - TDQ_LOCK(tdq); - } + TDQ_UNLOCK(tdn); spinlock_exit(); #endif return (TDQ_LOCKPTR(tdn));