Messages in this thread | | | Date | Tue, 20 May 2014 11:39:55 -0700 | Subject | Re: hanging aio process | From | Anatol Pomozov <> |
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Hi
On Tue, May 20, 2014 at 11:09 AM, Sebastian Ott <sebott@linux.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@linux.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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