Messages in this thread Patch in this message | | | From | Waiman Long <> | Subject | [PATCH] locking/rwsem: Synchronize task state & waiter->task of readers | Date | Tue, 10 Apr 2018 13:22:18 -0400 |
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It was observed occasionally in PowerPC systems that there was reader who had not been woken up but that its waiter->task had been cleared.
One probable cause of this missed wakeup may be the fact that the waiter->task and the task state have not been properly synchronized as the lock release-acquire pair of different locks in the wakeup code path does not provide a full memory barrier guarantee. So smp_store_mb() is now used to set waiter->task to NULL to provide a proper memory barrier for synchronization.
Signed-off-by: Waiman Long <longman@redhat.com> --- kernel/locking/rwsem-xadd.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index e795908..b3c588c 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -209,6 +209,23 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, smp_store_release(&waiter->task, NULL); } + /* + * To avoid missed wakeup of reader, we need to make sure + * that task state and waiter->task are properly synchronized. + * + * wakeup sleep + * ------ ----- + * __rwsem_mark_wake: rwsem_down_read_failed*: + * [S] waiter->task [S] set_current_state(state) + * MB MB + * try_to_wake_up: + * [L] state [L] waiter->task + * + * For the wakeup path, the original lock release-acquire pair + * does not provide enough guarantee of proper synchronization. + */ + smp_mb(); + adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; if (list_empty(&sem->wait_list)) { /* hit end of list above */ -- 1.8.3.1
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