Re: [PATCH 2/4] io_uring: switch deferred task_work to an io_wq_work_list

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/27/24 16:37, Jens Axboe wrote:
> On 3/27/24 9:45 AM, Jens Axboe wrote:
>>> smp_mb(), see the comment below, and fwiw "_after_atomic" would not
>>> work.
>>
>> For this one, I think all we need to do is have the wq_list_empty()
>> check be fully stable. If we read:
>>
>> nr_wait = atomic_read(&ctx->cq_wait_nr);
>>
>> right before a waiter does:
>>
>> atomic_set(&ctx->cq_wait_nr, foo);
>> set_current_state(TASK_INTERRUPTIBLE);
>>
>> then we need to ensure that the "I have work" check in
>> io_cqring_wait_schedule() sees the work. The spin_unlock() has release
>> semantics, and the current READ_ONCE() for work check sbould be enough,
>> no?
> 
> To answer my own question - no, it's not enough. Let me think about this
> a bit.

Right, to my knowledge release does nothing for write; read;
ordering, and all ops after can leak before the barrier.

-- 
Pavel Begunkov