Re: hanging aio process

From: Sebastian Ott
Date: Tue May 20 2014 - 14:09:44 EST

Hi,

On Tue, 20 May 2014, Anatol Pomozov wrote:
> On Tue, May 20, 2014 at 6:16 AM, Sebastian Ott
> <sebott@xxxxxxxxxxxxxxxxxx> 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)
>
>

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