From: Michael Qiu Date: Wed, 3 Feb 2021 10:40:59 +0800 Subject: blockjob: Fix crash with IOthread when block commit after snapshot Git-commit: 076d467aacdf6dc5d01e2e61740b1795f2aec2f6 Currently, if guest has workloads, IO thread will acquire aio_context lock before do io_submit, it leads to segmentfault when do block commit after snapshot. Just like below: Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 0x7f7c7d91f700 (LWP 99907)] 0x00005576d0f65aab in bdrv_mirror_top_pwritev at ../block/mirror.c:1437 1437 ../block/mirror.c: No such file or directory. (gdb) p s->job $17 = (MirrorBlockJob *) 0x0 (gdb) p s->stop $18 = false Call trace of IO thread: 0 0x00005576d0f65aab in bdrv_mirror_top_pwritev at ../block/mirror.c:1437 1 0x00005576d0f7f3ab in bdrv_driver_pwritev at ../block/io.c:1174 2 0x00005576d0f8139d in bdrv_aligned_pwritev at ../block/io.c:1988 3 0x00005576d0f81b65 in bdrv_co_pwritev_part at ../block/io.c:2156 4 0x00005576d0f8e6b7 in blk_do_pwritev_part at ../block/block-backend.c:1260 5 0x00005576d0f8e84d in blk_aio_write_entry at ../block/block-backend.c:1476 ... Switch to qemu main thread: 0 0x00007f903be704ed in __lll_lock_wait at /lib/../lib64/libpthread.so.0 1 0x00007f903be6bde6 in _L_lock_941 at /lib/../lib64/libpthread.so.0 2 0x00007f903be6bcdf in pthread_mutex_lock at /lib/../lib64/libpthread.so.0 3 0x0000564b21456889 in qemu_mutex_lock_impl at ../util/qemu-thread-posix.c:79 4 0x0000564b213af8a5 in block_job_add_bdrv at ../blockjob.c:224 5 0x0000564b213b00ad in block_job_create at ../blockjob.c:440 6 0x0000564b21357c0a in mirror_start_job at ../block/mirror.c:1622 7 0x0000564b2135a9af in commit_active_start at ../block/mirror.c:1867 8 0x0000564b2133d132 in qmp_block_commit at ../blockdev.c:2768 9 0x0000564b2141fef3 in qmp_marshal_block_commit at qapi/qapi-commands-block-core.c:346 10 0x0000564b214503c9 in do_qmp_dispatch_bh at ../qapi/qmp-dispatch.c:110 11 0x0000564b21451996 in aio_bh_poll at ../util/async.c:164 12 0x0000564b2146018e in aio_dispatch at ../util/aio-posix.c:381 13 0x0000564b2145187e in aio_ctx_dispatch at ../util/async.c:306 14 0x00007f9040239049 in g_main_context_dispatch at /lib/../lib64/libglib-2.0.so.0 15 0x0000564b21447368 in main_loop_wait at ../util/main-loop.c:232 16 0x0000564b21447368 in main_loop_wait at ../util/main-loop.c:255 17 0x0000564b21447368 in main_loop_wait at ../util/main-loop.c:531 18 0x0000564b212304e1 in qemu_main_loop at ../softmmu/runstate.c:721 19 0x0000564b20f7975e in main at ../softmmu/main.c:50 In IO thread when do bdrv_mirror_top_pwritev, the job is NULL, and stop field is false, this means the MirrorBDSOpaque "s" object has not been initialized yet, and this object is initialized by block_job_create(), but the initialize process is stuck in acquiring the lock. In this situation, IO thread come to bdrv_mirror_top_pwritev(),which means that mirror-top node is already inserted into block graph, but its bs->opaque->job is not initialized. The root cause is that qemu main thread do release/acquire when hold the lock, at the same time, IO thread get the lock after release stage, and the crash occured. Actually, in this situation, job->job.aio_context will not equal to qemu_get_aio_context(), and will be the same as bs->aio_context, thus, no need to release the lock, becasue bdrv_root_attach_child() will not change the context. This patch fix this issue. Fixes: 132ada80 "block: Adjust AioContexts when attaching nodes" Signed-off-by: Michael Qiu Message-Id: <20210203024059.52683-1-08005325@163.com> Signed-off-by: Kevin Wolf Signed-off-by: Bruce Rogers --- blockjob.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/blockjob.c b/blockjob.c index 98ac8af98299ac26c50ce5e4c5bb..62f1a537399f07a88ebf520eb560 100644 --- a/blockjob.c +++ b/blockjob.c @@ -212,15 +212,19 @@ int block_job_add_bdrv(BlockJob *job, const char *name, BlockDriverState *bs, uint64_t perm, uint64_t shared_perm, Error **errp) { BdrvChild *c; + bool need_context_ops; bdrv_ref(bs); - if (job->job.aio_context != qemu_get_aio_context()) { + + need_context_ops = bdrv_get_aio_context(bs) != job->job.aio_context; + + if (need_context_ops && job->job.aio_context != qemu_get_aio_context()) { aio_context_release(job->job.aio_context); } c = bdrv_root_attach_child(bs, name, &child_job, 0, job->job.aio_context, perm, shared_perm, job, errp); - if (job->job.aio_context != qemu_get_aio_context()) { + if (need_context_ops && job->job.aio_context != qemu_get_aio_context()) { aio_context_acquire(job->job.aio_context); } if (c == NULL) {