On Wed, 21 Jul 2021 12:59:39 +0200 Paolo Bonzini wrote: >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. Ah good catch. Hillf
