Re: hanging aio process
From: Sebastian Ott
Date: Wed May 21 2014 - 04:49:11 EST
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@xxxxxxxxxxxxxxxxxx> wrote:
> > > 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.
> >
> > 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
<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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