Re: [PATCH] blockjob: Fix crash with IOthread when block commit after snapshot

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

subject should start with [PATCH v5]

03.02.2021 05:40, 08005325@163.com wrote:
> From: Michael Qiu <qiudayu@huayun.com>
> 
> v5: reformat the commit log with backtrace of main thread
>      Add a boolean variable to make main thread could re-acquire
>      aio_context on success path.
> 
> v4: rebase to latest code
> 
> v3: reformat the commit log, remove duplicate content

patch history shouldn't go into commit message. So you should place it under '---' [*], after calling git format-patch

> 
> 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 <qiudayu@huayun.com>

I feel like there may be more problems (like the fact that drained section should be expanded, and
that expanding doesn't help as Michael said), but I think that temporary releasing locks is unsafe
thing, and if we can avoid it for some cases it's good, especially if it fixes some bug:

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

> ---

[*] patch history and anything that you don't want to put into final commit message goes here.

>   blockjob.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/blockjob.c b/blockjob.c
> index db3a21699c..d9dca36f65 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -212,15 +212,21 @@ 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;

I'd also put the second condition into same variable, just for less typing. Still it should work as is.

> +
> +    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) {
> 


-- 
Best regards,
Vladimir