Re: hanging aio process
From: Anatol Pomozov
Date: Tue May 20 2014 - 12:12:17 EST
Hi
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? 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.
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?
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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