[Qemu-devel] [PATCH] qemu-coroutine: segfault when restarting co_queue

[Bin Wu]

From: Bin Wu <address@hidden>

We tested VMs migration with their disk images by drive_mirror. With 
migration, two VMs copyed large files between each other. During the
test, a segfault occured. The stack was as follow:

(gdb) bt
#0  0x00007fa5a0c63fc5 in qemu_co_queue_run_restart (co=0x7fa5a1798648) at
qemu-coroutine-lock.c:66
#1  0x00007fa5a0c63bed in coroutine_swap (from=0x7fa5a178f160,
to=0x7fa5a1798648) at qemu-coroutine.c:97
#2  0x00007fa5a0c63dbf in qemu_coroutine_yield () at qemu-coroutine.c:140
#3  0x00007fa5a0c9e474 in nbd_co_receive_reply (s=0x7fa5a1a3cfd0,
request=0x7fa28c2ffa10, reply=0x7fa28c2ffa30, qiov=0x0, offset=0) at
block/nbd-client.c:165
#4  0x00007fa5a0c9e8b5 in nbd_co_writev_1 (client=0x7fa5a1a3cfd0,
sector_num=8552704, nb_sectors=2040, qiov=0x7fa5a1757468, offset=0) at
block/nbd-client.c:262
#5  0x00007fa5a0c9e9dd in nbd_client_session_co_writev (client=0x7fa5a1a3cfd0,
sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468) at
block/nbd-client.c:296
#6  0x00007fa5a0c9dda1 in nbd_co_writev (bs=0x7fa5a198fcb0, sector_num=8552704,
nb_sectors=2048, qiov=0x7fa5a1757468) at block/nbd.c:291
#7  0x00007fa5a0c509a4 in bdrv_aligned_pwritev (bs=0x7fa5a198fcb0,
req=0x7fa28c2ffbb0, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
flags=0) at block.c:3321
#8  0x00007fa5a0c50f3f in bdrv_co_do_pwritev (bs=0x7fa5a198fcb0,
offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3447
#9  0x00007fa5a0c51007 in bdrv_co_do_writev (bs=0x7fa5a198fcb0,
sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3471
#10 0x00007fa5a0c51074 in bdrv_co_writev (bs=0x7fa5a198fcb0, sector_num=8552704,
nb_sectors=2048, qiov=0x7fa5a1757468) at block.c:3480
#11 0x00007fa5a0c652ec in raw_co_writev (bs=0x7fa5a198c110, sector_num=8552704,
nb_sectors=2048, qiov=0x7fa5a1757468) at block/raw_bsd.c:62
#12 0x00007fa5a0c509a4 in bdrv_aligned_pwritev (bs=0x7fa5a198c110,
req=0x7fa28c2ffe30, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
flags=0) at block.c:3321
#13 0x00007fa5a0c50f3f in bdrv_co_do_pwritev (bs=0x7fa5a198c110,
offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3447
#14 0x00007fa5a0c51007 in bdrv_co_do_writev (bs=0x7fa5a198c110,
sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3471
#15 0x00007fa5a0c542b3 in bdrv_co_do_rw (opaque=0x7fa5a17a0000) at block.c:4706
#16 0x00007fa5a0c64e6e in coroutine_trampoline (i0=-1585909408, i1=32677) at
coroutine-ucontext.c:121
#17 0x00007fa59dc5aa50 in __correctly_grouped_prefixwc () from /lib64/libc.so.6
#18 0x0000000000000000 in ?? ()

After analyzing the stack and reviewing the code, we find the
qemu_co_queue_run_restart should not be put in the coroutine_swap function which
can be invoked by qemu_coroutine_enter or qemu_coroutine_yield. Only
qemu_coroutine_enter needs to restart the co_queue. The error scenario is given
in the following patch log.

Bin Wu (1):
  qemu-coroutine: fix qemu_co_queue_run_restart error

 qemu-coroutine.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

-- 
1.7.12.4