Re: [PATCH v2 2/2] io_uring: flush timeouts that should already have expired

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

On 19/12/2020 19:15, Marcelo Diop-Gonzalez wrote:
> Right now io_flush_timeouts() checks if the current number of events
> is equal to ->timeout.target_seq, but this will miss some timeouts if
> there have been more than 1 event added since the last time they were
> flushed (possible in io_submit_flush_completions(), for example). Fix
> it by recording the starting value of ->cached_cq_overflow -
> ->cq_timeouts instead of the target value, so that we can safely
> (without overflow problems) compare the number of events that have
> happened with the number of events needed to trigger the timeout.
> 
> Signed-off-by: Marcelo Diop-Gonzalez <marcelo827@gmail.com>
> ---
>  fs/io_uring.c | 30 +++++++++++++++++++++++-------
>  1 file changed, 23 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index f394bf358022..f62de0cb5fc4 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -444,7 +444,7 @@ struct io_cancel {
>  struct io_timeout {
>  	struct file			*file;
>  	u32				off;
> -	u32				target_seq;
> +	u32				start_seq;
>  	struct list_head		list;
>  	/* head of the link, used by linked timeouts only */
>  	struct io_kiocb			*head;
> @@ -1629,6 +1629,24 @@ static void __io_queue_deferred(struct io_ring_ctx *ctx)
>  	} while (!list_empty(&ctx->defer_list));
>  }
>  
> +static inline u32 io_timeout_events_left(struct io_kiocb *req)
> +{
> +	struct io_ring_ctx *ctx = req->ctx;
> +	u32 events;
> +
> +	/*
> +	 * events -= req->timeout.start_seq and the comparison between
> +	 * ->timeout.off and events will not overflow because each time
> +	 * ->cq_timeouts is incremented, ->cached_cq_tail is incremented too.
> +	 */
> +
> +	events = ctx->cached_cq_tail - atomic_read(&ctx->cq_timeouts);
> +	events -= req->timeout.start_seq;

It looks to me that events before the start_seq subtraction can have got wrapped
around start_seq.

e.g.
1) you submit a timeout with off=0xff...ff (start_seq=0 for convenience)

2) some time has passed, let @events = 0xff..ff - 1
so the timeout still waits

3) we commit 5 requests at once and call io_commit_cqring() only once for
them, so we get @events == 0xff..ff - 1 + 5, i.e. 4

@events == 4 < off == 0xff...ff,
so we didn't trigger out timeout even though should have

> +	if (req->timeout.off > events)
> +		return req->timeout.off - events;
> +	return 0;
> +}
> +
>  static void io_flush_timeouts(struct io_ring_ctx *ctx)
>  {
>  	while (!list_empty(&ctx->timeout_list)) {
> @@ -1637,8 +1655,7 @@ static void io_flush_timeouts(struct io_ring_ctx *ctx)
>  
>  		if (io_is_timeout_noseq(req))
>  			break;
> -		if (req->timeout.target_seq != ctx->cached_cq_tail
> -					- atomic_read(&ctx->cq_timeouts))
> +		if (io_timeout_events_left(req) > 0)
>  			break;
>  
>  		list_del_init(&req->timeout.list);
> @@ -5785,7 +5802,6 @@ static int io_timeout(struct io_kiocb *req)
>  	struct io_ring_ctx *ctx = req->ctx;
>  	struct io_timeout_data *data = req->async_data;
>  	struct list_head *entry;
> -	u32 tail, off = req->timeout.off;
>  
>  	spin_lock_irq(&ctx->completion_lock);
>  
> @@ -5799,8 +5815,8 @@ static int io_timeout(struct io_kiocb *req)
>  		goto add;
>  	}
>  
> -	tail = ctx->cached_cq_tail - atomic_read(&ctx->cq_timeouts);
> -	req->timeout.target_seq = tail + off;
> +	req->timeout.start_seq = ctx->cached_cq_tail -
> +		atomic_read(&ctx->cq_timeouts);
>  
>  	/*
>  	 * Insertion sort, ensuring the first entry in the list is always
> @@ -5813,7 +5829,7 @@ static int io_timeout(struct io_kiocb *req)
>  		if (io_is_timeout_noseq(nxt))
>  			continue;
>  		/* nxt.seq is behind @tail, otherwise would've been completed */
> -		if (off >= nxt->timeout.target_seq - tail)
> +		if (req->timeout.off >= io_timeout_events_left(nxt))
>  			break;
>  	}
>  add:
> 

-- 
Pavel Begunkov