Re: [PATCH v2 02/14] io_uring/cmd: fix tw <-> issue_flags conversion

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

On Sun, Mar 17, 2024 at 09:11:27PM -0600, Jens Axboe wrote:
> On 3/17/24 8:47 PM, Ming Lei wrote:
> > On Sun, Mar 17, 2024 at 08:40:59PM -0600, Jens Axboe wrote:
> >> On 3/17/24 8:32 PM, Pavel Begunkov wrote:
> >>> On 3/18/24 02:25, Jens Axboe wrote:
> >>>> On 3/17/24 8:23 PM, Ming Lei wrote:
> >>>>> On Mon, Mar 18, 2024 at 12:41:47AM +0000, Pavel Begunkov wrote:
> >>>>>> !IO_URING_F_UNLOCKED does not translate to availability of the deferred
> >>>>>> completion infra, IO_URING_F_COMPLETE_DEFER does, that what we should
> >>>>>> pass and look for to use io_req_complete_defer() and other variants.
> >>>>>>
> >>>>>> Luckily, it's not a real problem as two wrongs actually made it right,
> >>>>>> at least as far as io_uring_cmd_work() goes.
> >>>>>>
> >>>>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> >>>>>> Link: https://lore.kernel.org/r/eb08e72e837106963bc7bc7dccfd93d646cc7f36.1710514702.git.asml.silence@gmail.com
> >>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >>>
> >>> oops, I should've removed all the signed-offs
> >>>
> >>>>>> ---
> >>>>>>   io_uring/uring_cmd.c | 10 ++++++++--
> >>>>>>   1 file changed, 8 insertions(+), 2 deletions(-)
> >>>>>>
> >>>>>> diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
> >>>>>> index f197e8c22965..ec38a8d4836d 100644
> >>>>>> --- a/io_uring/uring_cmd.c
> >>>>>> +++ b/io_uring/uring_cmd.c
> >>>>>> @@ -56,7 +56,11 @@ EXPORT_SYMBOL_GPL(io_uring_cmd_mark_cancelable);
> >>>>>>   static void io_uring_cmd_work(struct io_kiocb *req, struct io_tw_state *ts)
> >>>>>>   {
> >>>>>>       struct io_uring_cmd *ioucmd = io_kiocb_to_cmd(req, struct io_uring_cmd);
> >>>>>> -    unsigned issue_flags = ts->locked ? 0 : IO_URING_F_UNLOCKED;
> >>>>>> +    unsigned issue_flags = IO_URING_F_UNLOCKED;
> >>>>>> +
> >>>>>> +    /* locked task_work executor checks the deffered list completion */
> >>>>>> +    if (ts->locked)
> >>>>>> +        issue_flags = IO_URING_F_COMPLETE_DEFER;
> >>>>>>         ioucmd->task_work_cb(ioucmd, issue_flags);
> >>>>>>   }
> >>>>>> @@ -100,7 +104,9 @@ void io_uring_cmd_done(struct io_uring_cmd *ioucmd, ssize_t ret, ssize_t res2,
> >>>>>>       if (req->ctx->flags & IORING_SETUP_IOPOLL) {
> >>>>>>           /* order with io_iopoll_req_issued() checking ->iopoll_complete */
> >>>>>>           smp_store_release(&req->iopoll_completed, 1);
> >>>>>> -    } else if (!(issue_flags & IO_URING_F_UNLOCKED)) {
> >>>>>> +    } else if (issue_flags & IO_URING_F_COMPLETE_DEFER) {
> >>>>>> +        if (WARN_ON_ONCE(issue_flags & IO_URING_F_UNLOCKED))
> >>>>>> +            return;
> >>>>>>           io_req_complete_defer(req);
> >>>>>>       } else {
> >>>>>>           req->io_task_work.func = io_req_task_complete;
> >>>>>
> >>>>> 'git-bisect' shows the reported warning starts from this patch.
> >>>
> >>> Thanks Ming
> >>>
> >>>>
> >>>> That does make sense, as probably:
> >>>>
> >>>> +    /* locked task_work executor checks the deffered list completion */
> >>>> +    if (ts->locked)
> >>>> +        issue_flags = IO_URING_F_COMPLETE_DEFER;
> >>>>
> >>>> this assumption isn't true, and that would mess with the task management
> >>>> (which is in your oops).
> >>>
> >>> I'm missing it, how it's not true?
> >>>
> >>>
> >>> static void ctx_flush_and_put(struct io_ring_ctx *ctx, struct io_tw_state *ts)
> >>> {
> >>>     ...
> >>>     if (ts->locked) {
> >>>         io_submit_flush_completions(ctx);
> >>>         ...
> >>>     }
> >>> }
> >>>
> >>> static __cold void io_fallback_req_func(struct work_struct *work)
> >>> {
> >>>     ...
> >>>     mutex_lock(&ctx->uring_lock);
> >>>     llist_for_each_entry_safe(req, tmp, node, io_task_work.node)
> >>>         req->io_task_work.func(req, &ts);
> >>>     io_submit_flush_completions(ctx);
> >>>     mutex_unlock(&ctx->uring_lock);
> >>>     ...
> >>> }
> >>
> >> I took a look too, and don't immediately see it. Those are also the two
> >> only cases I found, and before the patches, looks fine too.
> >>
> >> So no immediate answer there... But I can confirm that before this
> >> patch, test passes fine. With the patch, it goes boom pretty quick.
> >> Either directly off putting the task, or an unrelated memory crash
> >> instead.
> > 
> > In ublk, the translated 'issue_flags' is passed to io_uring_cmd_done()
> > from ioucmd->task_work_cb()(__ublk_rq_task_work()). That might be
> > related with the reason.
> 
> Or maybe ublk is doing multiple invocations of task_work completions? I
> added this:

Yes, your debug log & point does help.

This patch convert zero flag(!IO_URING_F_UNLOCKED) into IO_URING_F_COMPLETE_DEFER,
and somewhere is easily ignored, and follows the fix, which need to be
folded into patch 2.

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 5d4b448fdc50..22f2b52390a9 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3259,7 +3259,8 @@ static bool io_uring_try_cancel_uring_cmd(struct io_ring_ctx *ctx,
                        /* ->sqe isn't available if no async data */
                        if (!req_has_async_data(req))
                                cmd->sqe = NULL;
-                       file->f_op->uring_cmd(cmd, IO_URING_F_CANCEL);
+                       file->f_op->uring_cmd(cmd, IO_URING_F_CANCEL |
+                                       IO_URING_F_COMPLETE_DEFER);
                        ret = true;
                }
        }


Thanks,
Ming