Re: [PATCH v2 02/14] io_uring/cmd: fix tw <-> issue_flags conversion

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

On 3/18/24 02:25, Jens Axboe wrote:
> On 3/17/24 8:23 PM, Ming Lei wrote:
>> On Mon, Mar 18, 2024 at 12:41:47AM +0000, Pavel Begunkov wrote:
>>> !IO_URING_F_UNLOCKED does not translate to availability of the deferred
>>> completion infra, IO_URING_F_COMPLETE_DEFER does, that what we should
>>> pass and look for to use io_req_complete_defer() and other variants.
>>>
>>> Luckily, it's not a real problem as two wrongs actually made it right,
>>> at least as far as io_uring_cmd_work() goes.
>>>
>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>> Link: https://lore.kernel.org/r/eb08e72e837106963bc7bc7dccfd93d646cc7f36.1710514702.git.asml.silence@gmail.com
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>

oops, I should've removed all the signed-offs

>>> ---
>>>   io_uring/uring_cmd.c | 10 ++++++++--
>>>   1 file changed, 8 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
>>> index f197e8c22965..ec38a8d4836d 100644
>>> --- a/io_uring/uring_cmd.c
>>> +++ b/io_uring/uring_cmd.c
>>> @@ -56,7 +56,11 @@ EXPORT_SYMBOL_GPL(io_uring_cmd_mark_cancelable);
>>>   static void io_uring_cmd_work(struct io_kiocb *req, struct io_tw_state *ts)
>>>   {
>>>   	struct io_uring_cmd *ioucmd = io_kiocb_to_cmd(req, struct io_uring_cmd);
>>> -	unsigned issue_flags = ts->locked ? 0 : IO_URING_F_UNLOCKED;
>>> +	unsigned issue_flags = IO_URING_F_UNLOCKED;
>>> +
>>> +	/* locked task_work executor checks the deffered list completion */
>>> +	if (ts->locked)
>>> +		issue_flags = IO_URING_F_COMPLETE_DEFER;
>>>   
>>>   	ioucmd->task_work_cb(ioucmd, issue_flags);
>>>   }
>>> @@ -100,7 +104,9 @@ void io_uring_cmd_done(struct io_uring_cmd *ioucmd, ssize_t ret, ssize_t res2,
>>>   	if (req->ctx->flags & IORING_SETUP_IOPOLL) {
>>>   		/* order with io_iopoll_req_issued() checking ->iopoll_complete */
>>>   		smp_store_release(&req->iopoll_completed, 1);
>>> -	} else if (!(issue_flags & IO_URING_F_UNLOCKED)) {
>>> +	} else if (issue_flags & IO_URING_F_COMPLETE_DEFER) {
>>> +		if (WARN_ON_ONCE(issue_flags & IO_URING_F_UNLOCKED))
>>> +			return;
>>>   		io_req_complete_defer(req);
>>>   	} else {
>>>   		req->io_task_work.func = io_req_task_complete;
>>
>> 'git-bisect' shows the reported warning starts from this patch.

Thanks Ming

> 
> That does make sense, as probably:
> 
> +	/* locked task_work executor checks the deffered list completion */
> +	if (ts->locked)
> +		issue_flags = IO_URING_F_COMPLETE_DEFER;
> 
> this assumption isn't true, and that would mess with the task management
> (which is in your oops).

I'm missing it, how it's not true?


static void ctx_flush_and_put(struct io_ring_ctx *ctx, struct io_tw_state *ts)
{
	...
	if (ts->locked) {
		io_submit_flush_completions(ctx);
		...
	}
}

static __cold void io_fallback_req_func(struct work_struct *work)
{
	...
	mutex_lock(&ctx->uring_lock);
	llist_for_each_entry_safe(req, tmp, node, io_task_work.node)
		req->io_task_work.func(req, &ts);
	io_submit_flush_completions(ctx);
	mutex_unlock(&ctx->uring_lock);
	...
}

-- 
Pavel Begunkov