Re: [PATCH] io_uring: fix IO hang in io_wq_put_and_exit from do_exit()

[Ming Lei <ming.lei@redhat.com>]

On Fri, Sep 01, 2023 at 09:50:02AM +0800, Chengming Zhou wrote:
> On 2023/8/31 15:42, Ming Lei wrote:
> > io_wq_put_and_exit() is called from do_exit(), but all requests in io_wq
> > aren't cancelled in io_uring_cancel_generic() called from do_exit().
> > Meantime io_wq IO code path may share resource with normal iopoll code
> > path.
> > 
> > So if any HIPRI request is pending in io_wq_submit_work(), this request
> > may not get resouce for moving on, given iopoll isn't possible in
> > io_wq_put_and_exit().
> > 
> > The issue can be triggered when terminating 't/io_uring -n4 /dev/nullb0'
> > with default null_blk parameters.
> > 
> > Fix it by always cancelling all requests in io_wq from io_uring_cancel_generic(),
> > and this way is reasonable because io_wq destroying follows cancelling
> > requests immediately. Based on one patch from Chengming.
> 
> Thanks much for this work, I'm still learning these code, so maybe some
> silly questions below.
> 
> > 
> > Closes: https://lore.kernel.org/linux-block/3893581.1691785261@warthog.procyon.org.uk/
> > Reported-by: David Howells <dhowells@redhat.com>
> > Cc: Chengming Zhou <zhouchengming@bytedance.com>,
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  io_uring/io_uring.c | 40 ++++++++++++++++++++++++++++------------
> >  1 file changed, 28 insertions(+), 12 deletions(-)
> > 
> > diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> > index e7675355048d..18d5ab969c29 100644
> > --- a/io_uring/io_uring.c
> > +++ b/io_uring/io_uring.c
> > @@ -144,7 +144,7 @@ struct io_defer_entry {
> >  
> >  static bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
> >  					 struct task_struct *task,
> > -					 bool cancel_all);
> > +					 bool cancel_all, bool *wq_cancelled);
> >  
> >  static void io_queue_sqe(struct io_kiocb *req);
> >  
> > @@ -3049,7 +3049,7 @@ static __cold void io_ring_exit_work(struct work_struct *work)
> >  		if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
> >  			io_move_task_work_from_local(ctx);
> >  
> > -		while (io_uring_try_cancel_requests(ctx, NULL, true))
> > +		while (io_uring_try_cancel_requests(ctx, NULL, true, NULL))
> >  			cond_resched();
> >  
> >  		if (ctx->sq_data) {
> > @@ -3231,12 +3231,13 @@ static __cold bool io_uring_try_cancel_iowq(struct io_ring_ctx *ctx)
> >  
> >  static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
> >  						struct task_struct *task,
> > -						bool cancel_all)
> > +						bool cancel_all, bool *wq_cancelled)
> >  {
> > -	struct io_task_cancel cancel = { .task = task, .all = cancel_all, };
> > +	struct io_task_cancel cancel = { .task = task, .all = true, };
> >  	struct io_uring_task *tctx = task ? task->io_uring : NULL;
> >  	enum io_wq_cancel cret;
> >  	bool ret = false;
> > +	bool wq_active = false;
> >  
> >  	/* set it so io_req_local_work_add() would wake us up */
> >  	if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
> > @@ -3249,7 +3250,7 @@ static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
> >  		return false;
> >  
> >  	if (!task) {
> > -		ret |= io_uring_try_cancel_iowq(ctx);
> > +		wq_active = io_uring_try_cancel_iowq(ctx);
> >  	} else if (tctx && tctx->io_wq) {
> >  		/*
> >  		 * Cancels requests of all rings, not only @ctx, but
> > @@ -3257,11 +3258,20 @@ static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
> >  		 */
> >  		cret = io_wq_cancel_cb(tctx->io_wq, io_cancel_task_cb,
> >  				       &cancel, true);
> > -		ret |= (cret != IO_WQ_CANCEL_NOTFOUND);
> > +		wq_active = (cret != IO_WQ_CANCEL_NOTFOUND);
> >  	}
> > +	ret |= wq_active;
> > +	if (wq_cancelled)
> > +		*wq_cancelled = !wq_active;
> 
> Here it seems "wq_cancelled" means no any pending or running work anymore.

wq_cancelled means all requests in io_wq are canceled.

> 
> Why not just use the return value "loop", instead of using this new "wq_cancelled"?
> 
> If return value "loop" is true, we know there is still any request need to cancel,
> so we will loop the cancel process until there is no any request.
> 
> Ah, I guess you may want to cover one case: !wq_active && loop == true

If we just reply on 'loop', things could be like passing 'cancel_all' as
true, that might be over-kill. And I am still not sure why not canceling
all requests(cancel_all is true) in do_exit()?

But here it is enough to cancel all requests in io_wq only for solving
this IO hang issue.

> 
> >  
> > -	/* SQPOLL thread does its own polling */
> > -	if ((!(ctx->flags & IORING_SETUP_SQPOLL) && cancel_all) ||
> > +	/*
> > +	 * SQPOLL thread does its own polling
> > +	 *
> > +	 * io_wq may share IO resources(such as requests) with iopoll, so
> > +	 * iopoll requests have to be reapped for providing forward
> > +	 * progress to io_wq cancelling
> > +	 */
> > +	if (!(ctx->flags & IORING_SETUP_SQPOLL) ||
> >  	    (ctx->sq_data && ctx->sq_data->thread == current)) {
> >  		while (!wq_list_empty(&ctx->iopoll_list)) {
> >  			io_iopoll_try_reap_events(ctx);
> > @@ -3313,11 +3323,12 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
> >  	atomic_inc(&tctx->in_cancel);
> >  	do {
> >  		bool loop = false;
> > +		bool wq_cancelled;
> >  
> >  		io_uring_drop_tctx_refs(current);
> >  		/* read completions before cancelations */
> >  		inflight = tctx_inflight(tctx, !cancel_all);
> > -		if (!inflight)
> > +		if (!inflight && !tctx->io_wq)
> >  			break;
> >  
> 
> I think this inflight check should put after the cancel loop, because the
> cancel loop make sure there is no any request need to cancel, then we can
> loop inflight checking to make sure all inflight requests to complete.

But it is fine to break immediately in case that (!inflight && !tctx->io_wq) is true.


Thanks, 
Ming