Re: [PATCH V2] io_uring: consider the overflow of sequence for timeout req

[Jens Axboe <axboe@kernel.dk>]

On 10/14/19 5:51 AM, yangerkun wrote:
> The sequence for timeout req may overflow, and it will lead to wrong
> order of timeout req list. And we should consider two situation:
> 
> 1. ctx->cached_sq_head + count - 1 may overflow;
> 2. cached_sq_head of now may overflow compare with before
> cached_sq_head.
> 
> Fix the wrong logic by add record of count and use type long long to
> record the overflow.
> 
> Signed-off-by: yangerkun <yangerkun@huawei.com>
> ---
>   fs/io_uring.c | 31 +++++++++++++++++++++++++------
>   1 file changed, 25 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 76fdbe84aff5..c8dbf85c1c91 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -288,6 +288,7 @@ struct io_poll_iocb {
>   struct io_timeout {
>   	struct file			*file;
>   	struct hrtimer			timer;
> +	unsigned			count;
>   };

Can we reuse io_kiocb->submit->sequence for this? Unfortunately doing it
the way that you did, which does make the most logical sense, means that
struct io_kiocb will now spill into a 4th cacheline.

Or maybe fold ->sequence and ->submit.sequence to reclaim that space?

> @@ -1907,21 +1908,39 @@ static int io_timeout(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>   		count = 1;
>   
>   	req->sequence = ctx->cached_sq_head + count - 1;
> +	req->timeout.count = count;
>   	req->flags |= REQ_F_TIMEOUT;
>   
>   	/*
>   	 * Insertion sort, ensuring the first entry in the list is always
>   	 * the one we need first.
>   	 */
> -	tail_index = ctx->cached_cq_tail - ctx->rings->sq_dropped;
> -	req_dist = req->sequence - tail_index;
>   	spin_lock_irq(&ctx->completion_lock);
>   	list_for_each_prev(entry, &ctx->timeout_list) {
>   		struct io_kiocb *nxt = list_entry(entry, struct io_kiocb, list);
> -		unsigned dist;
> +		unsigned nxt_sq_head;
> +		long long tmp, tmp_nxt;
>   
> -		dist = nxt->sequence - tail_index;
> -		if (req_dist >= dist)
> +		/* count bigger than before should break directly. */
> +		if (count >= nxt->timeout.count)
> +			break;

Took me a bit, but I guess that's true. It's an optimization, maybe it'd be
cleaner if we just stuck to the sequence checking?

-- 
Jens Axboe