Re: hanging aio process
From: Sebastian Ott
Date: Wed May 21 2014 - 10:58:15 EST
On Wed, 21 May 2014, Anatol Pomozov wrote:
> On Wed, May 21, 2014 at 1:48 AM, Sebastian Ott
> <sebott@xxxxxxxxxxxxxxxxxx> 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@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
>
> 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.
I (hopefully) traced all gets and puts (of ctx->reqs). Hm gets and puts
are equal in numbers:
cat /sys/kernel/debug/tracing/trace | grep aio_complete | wc
49120 294720 3487520
cat /sys/kernel/debug/tracing/trace | grep do_io_submit | wc
49120 294720 3487520
cat /sys/kernel/debug/tracing/trace | grep -v aio_complete | grep -v do_io_submit
# tracer: function
#
# entries-in-buffer/entries-written: 98273/98273 #P:4
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
fio-793 [002] .... 86.697232: SyS_io_setup: get_reqs
fio-794 [000] .... 86.697232: SyS_io_setup: get_reqs
fio-788 [003] .... 86.697235: SyS_io_setup: get_reqs
fio-795 [001] .... 86.697237: SyS_io_setup: get_reqs
fio-793 [000] .... 87.217093: SyS_io_destroy: 00000000796d0000
fio-793 [000] .... 87.217095: kill_ioctx <-SyS_io_destroy
fio-795 [003] .... 87.221408: SyS_io_destroy: 000000007a038000
fio-795 [003] .... 87.221409: kill_ioctx <-SyS_io_destroy
fio-788 [003] .... 87.226725: SyS_io_destroy: 00000000796c8000
fio-788 [003] .... 87.226726: kill_ioctx <-SyS_io_destroy
fio-794 [000] .... 87.227840: SyS_io_destroy: 0000000079420000
fio-794 [000] .... 87.227841: kill_ioctx <-SyS_io_destroy
ksoftirqd/3-19 [003] ..s. 87.244201: free_ioctx_users <-percpu_ref_kill_rcu
ksoftirqd/3-19 [003] ..s. 87.244201: free_ioctx_users: put_reqs
ksoftirqd/3-19 [003] ..s. 87.244205: free_ioctx_users <-percpu_ref_kill_rcu
ksoftirqd/3-19 [003] ..s. 87.244205: free_ioctx_users: put_reqs
ksoftirqd/0-3 [000] ..s. 87.251898: free_ioctx_users <-percpu_ref_kill_rcu
ksoftirqd/0-3 [000] ..s. 87.251898: free_ioctx_users: put_reqs
ksoftirqd/0-3 [000] ..s. 87.251903: free_ioctx_users <-percpu_ref_kill_rcu
ksoftirqd/0-3 [000] ..s. 87.251903: free_ioctx_users: put_reqs
ksoftirqd/0-3 [000] ..s. 87.271868: aio_confirm_reqs: 00000000796d0000 reqs=1
ksoftirqd/0-3 [000] ..s. 87.271868: free_ioctx_reqs <-percpu_ref_kill_rcu
ksoftirqd/0-3 [000] ..s. 87.271874: aio_confirm_reqs: 0000000079420000 reqs=6
kworker/0:1-53 [000] .... 87.271875: free_ioctx <-process_one_work
fio-793 [001] .... 87.271879: SyS_io_destroy: 00000000796d0000 done
ksoftirqd/3-19 [003] ..s. 87.271899: aio_confirm_reqs: 000000007a038000 reqs=1
ksoftirqd/3-19 [003] ..s. 87.271899: free_ioctx_reqs <-percpu_ref_kill_rcu
fio-795 [000] .... 87.271904: SyS_io_destroy: 000000007a038000 done
ksoftirqd/3-19 [003] ..s. 87.271905: aio_confirm_reqs: 00000000796c8000 reqs=1
ksoftirqd/3-19 [003] ..s. 87.271905: free_ioctx_reqs <-percpu_ref_kill_rcu
kworker/3:2-222 [003] .... 87.271909: free_ioctx <-process_one_work
kworker/3:2-222 [003] .... 87.271912: free_ioctx <-process_one_work
fio-788 [003] .... 87.271920: SyS_io_destroy: 00000000796c8000 done
Hm..this time 5 additional refs. Again I've never worked with these percpu
refs but would it make sense to look for gets after kills (or is this
valid)?
diff --git a/fs/aio.c b/fs/aio.c
index a0ed6c7..6f83c11 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 +550,8 @@ 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);
+ trace_printk("put_reqs\n");
percpu_ref_put(&ctx->reqs);
}
@@ -695,6 +703,7 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
spin_unlock(&aio_nr_lock);
percpu_ref_get(&ctx->users); /* io_setup() will drop this ref */
+ trace_printk("get_reqs\n");
percpu_ref_get(&ctx->reqs); /* free_ioctx_users() will drop this */
err = ioctx_add_table(ctx, mm);
@@ -888,6 +897,7 @@ static inline struct kiocb *aio_get_req(struct kioctx *ctx)
if (unlikely(!req))
goto out_put;
+ trace_printk("get_reqs\n");
percpu_ref_get(&ctx->reqs);
req->ki_ctx = ctx;
@@ -1033,6 +1043,7 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
if (waitqueue_active(&ctx->wait))
wake_up(&ctx->wait);
+ trace_printk("put_reqs\n");
percpu_ref_put(&ctx->reqs);
}
EXPORT_SYMBOL(aio_complete);
@@ -1220,6 +1231,8 @@ 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 +1245,7 @@ SYSCALL_DEFINE1(io_destroy, aio_context_t, ctx)
* is destroyed.
*/
wait_for_completion(&requests_done);
+ trace_printk("%p done\n", ioctx);
return 0;
}
@@ -1457,6 +1471,7 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
return 0;
out_put_req:
put_reqs_available(ctx, 1);
+ trace_printk("put_reqs\n");
percpu_ref_put(&ctx->reqs);
kiocb_free(req);
return ret;
@@ -1512,6 +1527,10 @@ long do_io_submit(aio_context_t ctx_id, long nr,
blk_finish_plug(&plug);
percpu_ref_put(&ctx->users);
+
+ if (i != nr)
+ pr_warn("nr=%d i=%d\n", nr, i);
+
return i ? i : ret;
}
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