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Date: Tue, 20 May 2014 11:39:55 -0700
From: Anatol Pomozov <anatol.pomozov@...il.com>
To: Sebastian Ott <sebott@...ux.vnet.ibm.com>
Cc: Benjamin LaHaise <bcrl@...ck.org>, linux-aio@...ck.org,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: hanging aio process
Hi
On Tue, May 20, 2014 at 11:09 AM, Sebastian Ott
<sebott@...ux.vnet.ibm.com> wrote:
> Hi,
>
> On Tue, 20 May 2014, Anatol Pomozov wrote:
>> On Tue, May 20, 2014 at 6:16 AM, Sebastian Ott
>> <sebott@...ux.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.
Could you add atomic_read(&ctx->reqs.count) *after* the
percpu_ref_kill(&ctx->reqs)?
before percpu_ref_kill() the refcounter is per-CPU one and correctly
calculate the real value you need to take &ctx->reqs.pcpu_count into
account. But after percpu_ref_kill() the reqs.count become the normal
shared refcounter.
>
>>
>> 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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