Re: [Qemu-devel] [PATCH] linux-aio: avoid deadlock in nested aio_poll() calls

["Marcin Gibuła" <m.gibula@beyond.pl>]

On 04.08.2014 17:56, Stefan Hajnoczi wrote:
> If two Linux AIO request completions are fetched in the same
> io_getevents() call, QEMU will deadlock if request A's callback waits
> for request B to complete using an aio_poll() loop.  This was reported
> to happen with the mirror blockjob.
>
> This patch moves completion processing into a BH and makes it resumable.
> Nested event loops can resume completion processing so that request B
> will complete and the deadlock will not occur.
>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Ming Lei <ming.lei@canonical.com>
> Cc: Marcin Gibuła <m.gibula@beyond.pl>
> Reported-by: Marcin Gibuła <m.gibula@beyond.pl>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Still hangs...

Backtrace still looks like this:

Thread 1 (Thread 0x7f3d5313a900 (LWP 17440)):
#0  0x00007f3d4f38f286 in ppoll () from /lib64/libc.so.6
#1  0x00007f3d5347465b in ppoll (__ss=0x0, __timeout=0x0, 
__nfds=<optimized out>, __fds=<optimized out>) at 
/usr/include/bits/poll2.h:77
#2  qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, 
timeout=<optimized out>)
     at 
/var/tmp/portage/app-emulation/qemu-2.1.0/work/qemu-2.1.0/qemu-timer.c:314
#3  0x00007f3d53475970 in aio_poll (ctx=ctx@entry=0x7f3d54270c00, 
blocking=blocking@entry=true)
     at 
/var/tmp/portage/app-emulation/qemu-2.1.0/work/qemu-2.1.0/aio-posix.c:250
#4  0x00007f3d534695e7 in bdrv_drain_all () at 
/var/tmp/portage/app-emulation/qemu-2.1.0/work/qemu-2.1.0/block.c:1924
#5  0x00007f3d5346fe1f in bdrv_close (bs=bs@entry=0x7f3d5579b340) at 
/var/tmp/portage/app-emulation/qemu-2.1.0/work/qemu-2.1.0/block.c:1820
#6  0x00007f3d53470047 in bdrv_delete (bs=0x7f3d5579b340) at 
/var/tmp/portage/app-emulation/qemu-2.1.0/work/qemu-2.1.0/block.c:2094
#7  bdrv_unref (bs=0x7f3d5579b340) at 
/var/tmp/portage/app-emulation/qemu-2.1.0/work/qemu-2.1.0/block.c:5376
#8  0x00007f3d5347030b in bdrv_drop_intermediate 
(active=active@entry=0x7f3d54635e20, top=top@entry=0x7f3d5579b340, 
base=base@entry=0x7f3d54d956b0,
     backing_file_str=0x7f3d54d95700 
"/mnt/nfs/volumes/7c13c27f-0c48-4676-b075-6e8a3325383e/3785abe6-d2df-49da-9cba-e15cfce8e2af.qcow2")
     at 
/var/tmp/portage/app-emulation/qemu-2.1.0/work/qemu-2.1.0/block.c:2643
#9  0x00007f3d5335121a in commit_run (opaque=0x7f3d545cdac0) at 
/var/tmp/portage/app-emulation/qemu-2.1.0/work/qemu-2.1.0/block/commit.c:145
#10 0x00007f3d5347ebca in coroutine_trampoline (i0=<optimized out>, 
i1=<optimized out>)
     at 
/var/tmp/portage/app-emulation/qemu-2.1.0/work/qemu-2.1.0/coroutine-ucontext.c:118
#11 0x00007f3d4f2f49f0 in ?? () from /lib64/libc.so.6
#12 0x00007fff27d5ef50 in ?? ()
#13 0x0000000000000000 in ?? ()


-- 
mg