Messages in this thread Patch in this message | | | Date | Tue, 18 Jul 2017 13:25:49 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] osq_lock: fix osq_lock queue corruption |
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I added a few pictures, just the text didn't want to make sense to me.
On Fri, Jul 14, 2017 at 07:17:56PM +0530, Prateek Sood wrote:
> 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 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.
Also updated the comment..
--- kernel/locking/osq_lock.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)
--- a/kernel/locking/osq_lock.c +++ b/kernel/locking/osq_lock.c @@ -109,6 +109,19 @@ bool osq_lock(struct optimistic_spin_que prev = decode_cpu(old); 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); /*
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