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

[yangerkun]

On 2019/10/15 4:10, Jens Axboe wrote:
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 <yanger...@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?

Yeah, prefer to reuse ->submit.sequence to dump the count. I have never 
thought about the cacheline before. Thanks a lot!


@@ -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?

It's a good idea and thanks for you suggestion! I will resend the patch 
soon!

Thanks,
Kun.