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)
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@ -1749,19 +1749,19 @@ sched_switch_migrate(struct tdq *tdq, struct thread *td, int flags)
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*/
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spinlock_enter();
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thread_block_switch(td); /* This releases the lock on tdq. */
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TDQ_LOCK(tdn);
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/*
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* Acquire both run-queue locks before placing the thread on the new
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* run-queue to avoid deadlocks created by placing a thread with a
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* blocked lock on the run-queue of a remote processor. The deadlock
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* occurs when a third processor attempts to lock the two queues in
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* question while the target processor is spinning with its own
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* run-queue lock held while waiting for the blocked lock to clear.
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*/
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tdq_lock_pair(tdn, tdq);
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tdq_add(tdn, td, flags);
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tdq_notify(tdn, td);
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/*
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* After we unlock tdn the new cpu still can't switch into this
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* thread until we've unblocked it in cpu_switch(). The lock
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* pointers may match in the case of HTT cores. Don't unlock here
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* or we can deadlock when the other CPU runs the IPI handler.
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*/
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if (TDQ_LOCKPTR(tdn) != TDQ_LOCKPTR(tdq)) {
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TDQ_UNLOCK(tdn);
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TDQ_LOCK(tdq);
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}
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TDQ_UNLOCK(tdn);
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spinlock_exit();
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#endif
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return (TDQ_LOCKPTR(tdn));
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