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Message-ID: <CAOMFOmXXHBicH38onWgVAWhEXe2sE_RMiJXi=iMjJB15o7mOLQ@mail.gmail.com>
Date: Wed, 21 May 2014 07:18:53 -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 Wed, May 21, 2014 at 1:48 AM, Sebastian Ott
<sebott@...ux.vnet.ibm.com> wrote:
> On Wed, 21 May 2014, Sebastian Ott wrote:
>> On Tue, 20 May 2014, Anatol Pomozov wrote:
>> > On Tue, May 20, 2014 at 11:09 AM, Sebastian Ott
>> > <sebott@...ux.vnet.ibm.com> wrote:
>> > > 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)?
>>
>> I already did that and it didn't change, always 1 + (1UL << 31) in all
>> cases, before and after percpu_ref_kill(&ctx->reqs). I'm not really
>> familiar with this percpu_ref stuff but it looks like the initial
>> reference is dropped asynchronous.
>
>
> cat /sys/kernel/debug/tracing/trace
> # tracer: function
> #
> # entries-in-buffer/entries-written: 25/25 #P:4
> #
> # _-----=> irqs-off
> # / _----=> need-resched
> # | / _---=> hardirq/softirq
> # || / _--=> preempt-depth
> # ||| / delay
> # TASK-PID CPU# |||| TIMESTAMP FUNCTION
> # | | | |||| | |
> fio-856 [001] .... 160.876428: SyS_io_destroy: 0000000074a18000
> fio-856 [001] .... 160.876430: kill_ioctx <-SyS_io_destroy
> fio-855 [000] .... 160.887737: SyS_io_destroy: 0000000074f40600
> fio-855 [000] .... 160.887738: kill_ioctx <-SyS_io_destroy
> fio-849 [001] ..s. 160.911948: free_ioctx_users <-percpu_ref_kill_rcu
> ksoftirqd/0-3 [000] ..s. 160.932488: free_ioctx_users <-percpu_ref_kill_rcu
> fio-854 [001] .... 160.938881: SyS_io_destroy: 0000000074ac0600
> fio-854 [001] .... 160.938881: kill_ioctx <-SyS_io_destroy
> ksoftirqd/1-11 [001] ..s. 160.942016: aio_confirm_reqs: 0000000074a18000 reqs=1
> ksoftirqd/1-11 [001] ..s. 160.942016: free_ioctx_reqs <-percpu_ref_kill_rcu
> kworker/1:2-465 [001] .... 160.942021: free_ioctx <-process_one_work
> fio-856 [002] .... 160.942033: SyS_io_destroy: 0000000074a18000 done
> fio-849 [002] .... 160.942641: SyS_io_destroy: 0000000074f28600
> fio-849 [002] .... 160.942641: kill_ioctx <-SyS_io_destroy
> ksoftirqd/1-11 [001] ..s. 160.961981: free_ioctx_users <-percpu_ref_kill_rcu
> <idle>-0 [000] .ns. 160.962010: aio_confirm_reqs: 0000000074f40600 reqs=1
> <idle>-0 [000] .ns. 160.962011: free_ioctx_reqs <-percpu_ref_kill_rcu
> kworker/0:0-4 [000] .... 160.962016: free_ioctx <-process_one_work
> fio-855 [001] .... 160.962017: SyS_io_destroy: 0000000074f40600 done
> ksoftirqd/2-15 [002] ..s. 160.971998: free_ioctx_users <-percpu_ref_kill_rcu
> ksoftirqd/1-11 [001] ..s. 160.994552: aio_confirm_reqs: 0000000074ac0600 reqs=2
Here it is. aio context 0000000074ac0600 has refcount == 2 (one for
initial refcount and one grabbed by someone). You need to find the one
who grabbed the refcount and figure out why it does not drop it.
> <idle>-0 [002] .ns. 161.061976: aio_confirm_reqs: 0000000074f28600 reqs=1
> <idle>-0 [002] .ns. 161.061976: free_ioctx_reqs <-percpu_ref_kill_rcu
> kworker/2:2-181 [002] .... 161.061980: free_ioctx <-process_one_work
> fio-849 [003] .... 161.061983: SyS_io_destroy: 0000000074f28600 done
>
>
> diff --git a/fs/aio.c b/fs/aio.c
> index a0ed6c7..3b8e989 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -521,6 +521,13 @@ static void free_ioctx_reqs(struct percpu_ref *ref)
> schedule_work(&ctx->free_work);
> }
>
> +void aio_confirm_reqs(struct percpu_ref *ref)
> +{
> + struct kioctx *ctx = container_of(ref, struct kioctx, reqs);
> +
> + trace_printk("%p reqs=%d\n", ctx, atomic_read(&ref->count));
> +}
> +
> /*
> * When this function runs, the kioctx has been removed from the "hash table"
> * and ctx->users has dropped to 0, so we know no more kiocbs can be submitted -
> @@ -543,7 +550,7 @@ static void free_ioctx_users(struct percpu_ref *ref)
>
> spin_unlock_irq(&ctx->ctx_lock);
>
> - percpu_ref_kill(&ctx->reqs);
> + percpu_ref_kill_and_confirm(&ctx->reqs, aio_confirm_reqs);
> percpu_ref_put(&ctx->reqs);
> }
>
> @@ -1220,6 +1227,8 @@ SYSCALL_DEFINE1(io_destroy, aio_context_t, ctx)
> struct completion requests_done =
> COMPLETION_INITIALIZER_ONSTACK(requests_done);
>
> + trace_printk("%p\n", ioctx);
> +
> /* Pass requests_done to kill_ioctx() where it can be set
> * in a thread-safe way. If we try to set it here then we have
> * a race condition if two io_destroy() called simultaneously.
> @@ -1232,6 +1241,7 @@ SYSCALL_DEFINE1(io_destroy, aio_context_t, ctx)
> * is destroyed.
> */
> wait_for_completion(&requests_done);
> + trace_printk("%p done\n", ioctx);
>
> return 0;
> }
>
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