Re: [PATCH 5.11] io_uring: fix wqe->lock/completion_lock deadlock

[Jens Axboe <axboe@kernel.dk>]

On 1/26/21 6:52 PM, Joseph Qi wrote:
> 
> 
> On 1/27/21 7:35 AM, Pavel Begunkov wrote:
>> Joseph reports following deadlock:
>>
>> CPU0:
>> ...
>> io_kill_linked_timeout  // &ctx->completion_lock
>> io_commit_cqring
>> __io_queue_deferred
>> __io_queue_async_work
>> io_wq_enqueue
>> io_wqe_enqueue  // &wqe->lock
>>
>> CPU1:
>> ...
>> __io_uring_files_cancel
>> io_wq_cancel_cb
>> io_wqe_cancel_pending_work  // &wqe->lock
>> io_cancel_task_cb  // &ctx->completion_lock
>>
>> Only __io_queue_deferred() calls queue_async_work() while holding
>> ctx->completion_lock, enqueue drained requests via io_req_task_queue()
>> instead.
>>
> We should follow &wqe->lock > &ctx->completion_lock from now on, right?
> I was thinking getting completion_lock first before:(
> 
> Moreover, there are so many locks and no suggested locking order in
> comments, so that it is hard for us to participate in the work.

So many locks? There's really not even a handful, and only a few that
have overlaps (and hence ordering). Other sub-systems are have a crap
ton more :-). But I can document the ordering, at least that's a start
even if things like that tend to go stale. 

Thanks for reporting and testing! I'll queue this up for 5.11.

-- 
Jens Axboe