Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()

[Hillf Danton <hdanton@sina.com>]

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);