Commit 50972fe7 authored by Prateek Sood's avatar Prateek Sood Committed by Ingo Molnar

locking/osq_lock: Fix osq_lock queue corruption

Fix ordering of link creation between node->prev and prev->next in
osq_lock(). A case in which the status of optimistic spin queue is
CPU6->CPU2 in which CPU6 has acquired the lock.

        tail
          v
  ,-. <- ,-.
  |6|    |2|
  `-' -> `-'

At this point if CPU0 comes in to acquire osq_lock, it will update the
tail count.

  CPU2			CPU0
  ----------------------------------

				       tail
				         v
			  ,-. <- ,-.    ,-.
			  |6|    |2|    |0|
			  `-' -> `-'    `-'

After tail count update if CPU2 starts to unqueue itself from
optimistic spin queue, it will find an updated tail count with CPU0 and
update CPU2 node->next to NULL in osq_wait_next().

  unqueue-A

	       tail
	         v
  ,-. <- ,-.    ,-.
  |6|    |2|    |0|
  `-'    `-'    `-'

  unqueue-B

  ->tail != curr && !node->next

If reordering of following stores happen then prev->next where prev
being CPU2 would be updated to point to CPU0 node:

				       tail
				         v
			  ,-. <- ,-.    ,-.
			  |6|    |2|    |0|
			  `-'    `-' -> `-'

  osq_wait_next()
    node->next <- 0
    xchg(node->next, NULL)

	       tail
	         v
  ,-. <- ,-.    ,-.
  |6|    |2|    |0|
  `-'    `-'    `-'

  unqueue-C

At this point if next instruction
	WRITE_ONCE(next->prev, prev);
in CPU2 path is committed before the update of CPU0 node->prev = prev then
CPU0 node->prev will point to CPU6 node.

	       tail
    v----------. v
  ,-. <- ,-.    ,-.
  |6|    |2|    |0|
  `-'    `-'    `-'
     `----------^

At this point if CPU0 path's node->prev = prev is committed resulting
in change of CPU0 prev back to CPU2 node. CPU2 node->next is NULL
currently,

				       tail
			                 v
			  ,-. <- ,-. <- ,-.
			  |6|    |2|    |0|
			  `-'    `-'    `-'
			     `----------^

so if CPU0 gets into unqueue path of osq_lock it will keep spinning
in infinite loop as condition prev->next == node will never be true.
Signed-off-by: default avatarPrateek Sood <prsood@codeaurora.org>
[ Added pictures, rewrote comments. ]
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: sramana@codeaurora.org
Link: http://lkml.kernel.org/r/1500040076-27626-1-git-send-email-prsood@codeaurora.orgSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 9d664c0a
...@@ -109,6 +109,19 @@ bool osq_lock(struct optimistic_spin_queue *lock) ...@@ -109,6 +109,19 @@ bool osq_lock(struct optimistic_spin_queue *lock)
prev = decode_cpu(old); prev = decode_cpu(old);
node->prev = prev; node->prev = prev;
/*
* osq_lock() unqueue
*
* node->prev = prev osq_wait_next()
* WMB MB
* prev->next = node next->prev = prev // unqueue-C
*
* Here 'node->prev' and 'next->prev' are the same variable and we need
* to ensure these stores happen in-order to avoid corrupting the list.
*/
smp_wmb();
WRITE_ONCE(prev->next, node); WRITE_ONCE(prev->next, node);
/* /*
......
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