Re: [PATCH 4/4] io_uring: cancel sqpoll via task_work

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

On 11/03/2021 23:29, Pavel Begunkov wrote:
> 1) The first problem is io_uring_cancel_sqpoll() ->
> io_uring_cancel_task_requests() basically doing park(); park(); and so
> hanging.
> 
> 2) Another one is more subtle, when the master task is doing cancellations,
> but SQPOLL task submits in-between the end of the cancellation but
> before finish() requests taking a ref to the ctx, and so eternally
> locking it up.
> 
> 3) Yet another is a dying SQPOLL task doing io_uring_cancel_sqpoll() and
> same io_uring_cancel_sqpoll() from the owner task, they race for
> tctx->wait events. And there probably more of them.
> 
> Instead do SQPOLL cancellations from within SQPOLL task context via
> task_work, see io_sqpoll_cancel_sync(). With that we don't need temporal
> park()/unpark() during cancellation, which is ugly, subtle and anyway
> doesn't allow to do io_run_task_work() properly.> 
> io_uring_cancel_sqpoll() is called only from SQPOLL task context and
> under sqd locking, so all parking is removed from there. And so,
> io_sq_thread_[un]park() and io_sq_thread_stop() are not used now by
> SQPOLL task, and that spare us from some headache.
> 
> Also remove ctx->sqd_list early to avoid 2). And kill tctx->sqpoll,
> which is not used anymore.


Looks, the chunk below somehow slipped from the patch. Not important
for 5.12, but can can be folded anyway

diff --git a/include/linux/io_uring.h b/include/linux/io_uring.h
index 9761a0ec9f95..c24c62b47745 100644
--- a/include/linux/io_uring.h
+++ b/include/linux/io_uring.h
@@ -22,7 +22,6 @@ struct io_uring_task {
 	void			*io_wq;
 	struct percpu_counter	inflight;
 	atomic_t		in_idle;
-	bool			sqpoll;
 
 	spinlock_t		task_lock;
 	struct io_wq_work_list	task_list;


-- 
Pavel Begunkov