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);
