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. At this point if CPU0
comes in to acquire osq_lock, it will update the tail count. 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(). If reordering of following stores happen then
prev->next where prev being CPU2 would be updated to point to CPU0 node:
	node->prev = prev;
	WRITE_ONCE(prev->next, node);

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. 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, 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.

Change-Id: I48d847096daf3c228de90ae1cd2a6415b7bde65a
Signed-off-by: Prateek Sood <prsood@codeaurora.org>

0 files changed, 0 insertions, 0 deletions