Re: hanging aio process

[Sebastian Ott <sebott@linux.vnet.ibm.com>]

Hi,

On Tue, 20 May 2014, Anatol Pomozov wrote:
> On Tue, May 20, 2014 at 6:16 AM, Sebastian Ott
> <sebott@linux.vnet.ibm.com> wrote:
> > On Tue, 20 May 2014, Sebastian Ott wrote:
> >> On Mon, 19 May 2014, Benjamin LaHaise wrote:
> >> > It is entirely possible the bug isn't
> >> > caused by the referenced commit, as the commit you're pointing to merely
> >> > makes io_destroy() syscall wait for all aio outstanding to complete
> >> > before returning.
> >>
> >> I cannot reproduce this when I revert said commit (on top of 14186fe). If
> >> that matters - the arch is s390.
> >
> > Hm, ok - maybe that commit is really just highlighting a refcounting bug.
> > I just compared traces for a good and a few bad cases. The good case:
> > # tracer: function
> > #
> > # entries-in-buffer/entries-written: 16/16   #P:4
> > #
> > #                              _-----=> irqs-off
> > #                             / _----=> need-resched
> > #                            | / _---=> hardirq/softirq
> > #                            || / _--=> preempt-depth
> > #                            ||| /     delay
> > #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
> > #              | |       |   ||||       |         |
> >              fio-732   [003] ....    17.989315: kill_ioctx <-SyS_io_destroy
> >              fio-739   [003] ....    18.000563: kill_ioctx <-SyS_io_destroy
> >      ksoftirqd/3-19    [003] ..s.    18.031673: free_ioctx_users <-percpu_ref_kill_rcu
> >      ksoftirqd/3-19    [003] ..s.    18.031679: free_ioctx_users <-percpu_ref_kill_rcu
> >              fio-737   [003] ....    18.038765: kill_ioctx <-SyS_io_destroy
> >      ksoftirqd/3-19    [003] ..s.    18.062488: free_ioctx_reqs <-percpu_ref_kill_rcu
> >      ksoftirqd/3-19    [003] ..s.    18.062494: free_ioctx_reqs <-percpu_ref_kill_rcu
> >      kworker/3:1-57    [003] ....    18.062499: free_ioctx <-process_one_work
> >      kworker/3:1-57    [003] ....    18.062506: free_ioctx <-process_one_work
> >      ksoftirqd/3-19    [003] ..s.    18.072275: free_ioctx_users <-percpu_ref_kill_rcu
> >              fio-738   [003] ....    18.102419: kill_ioctx <-SyS_io_destroy
> >           <idle>-0     [003] .ns.    18.111668: free_ioctx_reqs <-percpu_ref_kill_rcu
> >      kworker/3:1-57    [003] ....    18.111675: free_ioctx <-process_one_work
> >      ksoftirqd/3-19    [003] ..s.    18.138035: free_ioctx_users <-percpu_ref_kill_rcu
> >           <idle>-0     [003] .ns.    18.191665: free_ioctx_reqs <-percpu_ref_kill_rcu
> >      kworker/3:1-57    [003] ....    18.191671: free_ioctx <-process_one_work
> >
> > (4 fio workers, free_ioctx_reqs is called 4 times)
> >
> > One of the bad cases:
> > # tracer: function
> > #
> > # entries-in-buffer/entries-written: 14/14   #P:4
> > #
> > #                              _-----=> irqs-off
> > #                             / _----=> need-resched
> > #                            | / _---=> hardirq/softirq
> > #                            || / _--=> preempt-depth
> > #                            ||| /     delay
> > #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
> > #              | |       |   ||||       |         |
> >              fio-834   [000] ....    51.127359: kill_ioctx <-SyS_io_destroy
> >           <idle>-0     [000] ..s.    51.170237: free_ioctx_users <-percpu_ref_kill_rcu
> >              fio-828   [001] ....    51.189717: kill_ioctx <-SyS_io_destroy
> >              fio-833   [001] ..s.    51.220178: free_ioctx_users <-percpu_ref_kill_rcu
> >           <idle>-0     [000] .ns.    51.220230: free_ioctx_reqs <-percpu_ref_kill_rcu
> >      kworker/0:3-661   [000] ....    51.220238: free_ioctx <-process_one_work
> >           <idle>-0     [001] .ns.    51.260188: free_ioctx_reqs <-percpu_ref_kill_rcu
> >      kworker/1:2-103   [001] ....    51.260198: free_ioctx <-process_one_work
> >              fio-833   [002] ....    51.287602: kill_ioctx <-SyS_io_destroy
> >            udevd-868   [002] ..s1    51.332519: free_ioctx_users <-percpu_ref_kill_rcu
> >           <idle>-0     [002] .ns.    51.450180: free_ioctx_reqs <-percpu_ref_kill_rcu
> >      kworker/2:2-191   [002] ....    51.450191: free_ioctx <-process_one_work
> >              fio-835   [003] ....    51.907530: kill_ioctx <-SyS_io_destroy
> >      ksoftirqd/3-19    [003] ..s.    52.000232: free_ioctx_users <-percpu_ref_kill_rcu
> >
> > (1 fio worker in D state, free_ioctx_reqs is called 3 times)
> 
> 
> Looking at the second trace: the first 3 io_destroy() calls cause
> free_ioctx_reqs(), but the last one does not call free_ioctx_reqs().
> Do you have more logs after the last line?

Nope that was all.

> If there is no more
> free_ioctx_reqs() then it means something keeps ctx->reqs refcounter.
> I suggest to add some logging to kernel to figure out what is the
> refcount value at this moment.

Jep, I did that this morning (via atomic_read(&ctx->reqs.count) in
free_ioctx_users before percpu_ref_kill(&ctx->reqs); is called) and
the value was always the same
	1 + (1UL << 31)
even for the free_ioctx_users invocations that were not followed by
free_ioctx_reqs.

> 
> But *if* there was one more free_ioctx_reqs() that does not
> complete(ctx->requests_done) then it means the thread does not see
> value of requests_done field. It would make me believe there is a
> problem with memory reordering. I do not know whether s390 reorders
> stores, but anyway, could you please check this patch?

Nope s390 doesn't reorder stores - I tried the patch anyway, but it didn't
help.

Regards,
Sebastian
> 
> anatol:linux $ git diff
> diff --git a/fs/aio.c b/fs/aio.c
> index a0ed6c7..6e3c0bb 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -514,6 +514,7 @@ static void free_ioctx_reqs(struct percpu_ref *ref)
>         struct kioctx *ctx = container_of(ref, struct kioctx, reqs);
> 
>         /* At this point we know that there are no any in-flight requests */
> +       rmb();
>         if (ctx->requests_done)
>                 complete(ctx->requests_done);
> 
> @@ -758,6 +759,7 @@ static void kill_ioctx(struct mm_struct *mm,
> struct kioctx *ctx,
>                         vm_munmap(ctx->mmap_base, ctx->mmap_size);
> 
>                 ctx->requests_done = requests_done;
> +               wmb();
>                 percpu_ref_kill(&ctx->users);
>         } else {
>                 if (requests_done)
> 
>