On 21/07/21 12:11, Hillf Danton wrote:
On Wed, 21 Jul 2021 09:25:32 +0200 Thomas Gleixner wrote:
On Wed, Jul 21 2021 at 15:04, Hillf Danton wrote:
But the preempting waker can not make sense without the waiter who is bloody
special. Why is it so in the first place? Or it is not at all but the race
existing from Monday to Friday.
See the large comment in eventfd_poll().
Is it likely for a reader to make eventfd_poll() return 0?
read * poll write
---- * ----------------- ------------
* count = ctx->count (INVALID!)
* lock ctx->qwh.lock
* ctx->count += n
* **waitqueue_active is false**
* **no wake_up_locked_poll!**
* unlock ctx->qwh.lock
lock ctx->qwh.lock
*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
ctx->count -= *cnt;
**waitqueue_active is false**
unlock ctx->qwh.lock
* lock ctx->wqh.lock (in poll_wait)
* __add_wait_queue
* unlock ctx->wqh.lock
* eventfd_poll returns 0
*/
count = READ_ONCE(ctx->count);
No, it's simply impossible. The same comment explains why: "count =
ctx->count" cannot move above poll_wait's locking of ctx->wqh.lock.
Paolo
