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

[Marcelo Diop-Gonzalez <marcelo827@gmail.com>]

On Sat, Jan 2, 2021 at 2:57 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> 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,

Oof, good catch... I'll try to think about it some more. Feels like
there ought to
be a nice way to do it but maybe it's quite tricky :/

-Marcelo

> 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