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From: "Denis V. Lunev" <den@openvz.org>
Date: Wed, 17 Aug 2016 21:06:52 +0300
Message-Id: <1471457214-3994-1-git-send-email-den@openvz.org>
Subject: [Qemu-devel] [PATCH for 2.7 0/2] block: fixes for deadlock in flush
 code
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To: qemu-block@nongnu.org, qemu-devel@nongnu.org
Cc: den@openvz.org, Evgeny Yakovlev <eyakovlev@virtuozzo.com>, Stefan Hajnoczi <stefanha@redhat.com>, Fam Zheng <famz@redhat.com>, Kevin Wolf <kwolf@redhat.com>, Max Reitz <mreitz@redhat.com>

We have suffered from the following deadlock

Thread 2 (Thread 0x7f1b7edf9700 (LWP 240293)):
#0  0x00007f1bd1f0675f in ppoll () from /lib64/libc.so.6
#1  0x00007f1bd8c1d78b 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=timeout@entry=-1) at qemu-timer.c:310
#3  0x00007f1bd8c1e8bf in aio_poll (ctx=0x7f1bda091780, blocking=blocking@entry=true) at aio-posix.c:451
#4  0x00007f1bd8c119cf in bdrv_drain_one (bs=bs@entry=0x7f1bda0f2000) at block.c:2055
#5  0x00007f1bd8c13244 in bdrv_drain_all () at block.c:2115
#6  0x00007f1bd8a2c5e3 in vm_stop (state=<optimized out>) at /usr/src/debug/qemu-2.3.0/cpus.c:685
#7  0x00007f1bd8a2c636 in vm_stop_force_state (state=<optimized out>) at /usr/src/debug/qemu-2.3.0/cpus.c:1383
#8  0x00007f1bd8bc798f in migration_completion (start_time=<synthetic pointer>, old_vm_running=<synthetic pointer>, current_active_state=<optimized out>, s=0x7f1bd90e3c20 <current_migration.37255>)
        at migration/migration.c:1213
#9  migration_thread (opaque=0x7f1bd90e3c20 <current_migration.37255>) at migration/migration.c:1314
#10 0x00007f1bd21e3dc5 in start_thread () from /lib64/libpthread.so.0
#11 0x00007f1bd1f10ced in clone () from /lib64/libc.so.6

The problem was narrowed down to the commit
    commit 3ff2f67a7c24183fcbcfe1332e5223ac6f96438c
    Author: Evgeny Yakovlev <eyakovlev@virtuozzo.com>
    Date:   Mon Jul 18 22:39:52 2016 +0300
    block: ignore flush requests when storage is clean

This patches contains fixes for the situation. The probability of the
problem is not that big. Our regression testing faces it ~1 time
a week or less.

Signed-off-by: Evgeny Yakovlev <eyakovlev@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <famz@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>