Messages in this thread | | | Date | Thu, 12 Jan 2023 11:58:27 +0800 | Subject | Re: [PATCH v2 1/2] fscache: Use wait_on_bit() to wait for the freeing of relinquished volume | From | Jingbo Xu <> |
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On 1/12/23 12:06 AM, David Howells wrote: > Hou Tao <houtao@huaweicloud.com> wrote: > >> clear_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &cursor->flags); >> + /* >> + * Paired with barrier in wait_on_bit(). Check >> + * wake_up_bit() and waitqueue_active() for details. >> + */ >> + smp_mb__after_atomic(); >> wake_up_bit(&cursor->flags, FSCACHE_VOLUME_ACQUIRE_PENDING); > > What two values are you applying a partial ordering to?
Yeah Hou Tao has explained that a full barrier is needed here to avoid the potential reordering at the waker side.
As I was also researching on this these days, I'd like to share my thought on this, hopefully if it could give some insight :)
Without the barrier at the waker side, it may suffer from the following race:
``` CPU0 - waker CPU1 - waiter
if (waitqueue_active(wq_head)) <-- find no wq_entry in wq_head list wake_up(wq_head);
for (;;) { prepare_to_wait(...); # add wq_entry into wq_head list
if (@cond) <-- @cond is false break; schedule(); <-- wq_entry still in wq_head list, wait for next wakeup } finish_wait(&wq_head, &wait);
@cond = true; ```
in which case the waiter misses the wakeup for one time.
-- Thanks, Jingbo
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