RE: [PATCH v4] blockjob: Fix crash with IOthread when block commit after snapshot

[仇大玉]

I'm so sorry, forgive my mail client(outlook)

I have try your solution, It doesn't work, still cause crash.

The reason is:  we 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"

But the root cause is that in block_job_create() we released(unnecessary) the 
aio_context, and the iothread get the context.

Script has to part, one is run in the VM (to give some workload) we named 
script A:
#!/bin/sh
For((i=1;i<=100000000;i++));
Do
dd if=/dev/zero of=./xxx bs=1M count=200
sleep 6
done

Another one is in the hypervisor, we named script B:
#!/bin/sh
for((i=1;i<=10000000;i++));
do
virsh snapshot-create-as fqtest --atomic --no-metadata --name fq6 --disk-only  
--diskspec vda,snapshot=external,file=/home/michael/snapshot/fq6.qcow2;
virsh blockcommit fqtest /home/michael/snapshot/fq6.qcow2 --shallow --verbose 
--wait --pivot --top /home/michael/snapshot/fq6.qcow2;
rm -r fq6.qcow2
done

How to reproduce:
1. start a VM, my case is use libvirt, named fqtest
2. run script B in hypervisor
3. after guest boot up, login and run script A in vda.

Make sure, the IO thread enabled for vda.

Mostly, just wait for several minutes, it will crash. 

The whole thread backtrace is:

[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

(gdb) bt
#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
#6  0x00005576d1060ddb in coroutine_trampoline at 
../util/coroutine-ucontext.c:173
#7  0x00007f7c8d3be0d0 in __start_context at /lib/../lib64/libc.so.6
#8  0x00007f7b52beb1e0 in  ()
#9  0x0000000000000000 in  ()

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

Thanks,
Michael

-----Original Message-----
From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> 
Sent: 2021年2月1日 18:28
To: 08005325@163.com; kwolf@redhat.com; mreitz@redhat.com; jsnow@redhat.com
Cc: 仇大玉 <qiudayu@huayun.com>; qemu-devel@nongnu.org; qemu-block@nongnu.org
Subject: Re: [PATCH v4] blockjob: Fix crash with IOthread when block commit 
after snapshot

Hi!

Tanks for fixing and sorry for a delay!

Please send each new version of a patch as a separate branch. It's a rule from 
https://wiki.qemu.org/Contribute/SubmitAPatch and it is more readable and less 
probable that your patch will be missed.

28.01.2021 04:30, 08005325@163.com wrote:
> From: Michael Qiu <qiudayu@huayun.com>
> 
> v4: rebase to latest code
> 
> v3: reformat the commit log, remove duplicate content
> 
> v2: modify the coredump backtrace within commit log with the newest
>      qemu with master branch

Such things shouldn't be in a commit message. You may put such comments after 
--- line[*] in a generated patch email

> 
> 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:

Do you have some reproducer script?

> 
> 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
> 
> (gdb) bt
> 
> Switch to qemu main thread:
> /lib/../lib64/libpthread.so.0
> /lib/../lib64/libpthread.so.0
> ../util/qemu-thread-posix.c:79
> qapi/qapi-commands-block-core.c:346
> ../qapi/qmp-dispatch.c:110
> /lib/../lib64/libglib-2.0.so.0

Not very informative bt..

> 
> 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.

Could you show another thread bt?

Hm, so you argue that we come to bdrv_mirror_top_pwritev() (which means that 
mirror-top node is already inserted into block graph), but its bs->opaque is 
not initialized?

Hmm, really in mirror_start_job we do insert mirror_top_bs before 
block_job_create() call.

But we should do that all in a drained section, so that no parallel io requests 
may come.

And we have a drained section but it finishes immediately after bdrv_append, 
when bs_opaque is still not initialized. Probably we just need to expand it?


  May be:

diff --git a/block/mirror.c b/block/mirror.c index 8e1ad6eceb..0a6bfc1230 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1610,11 +1610,11 @@ static BlockJob *mirror_start_job(
      bdrv_ref(mirror_top_bs);
      bdrv_drained_begin(bs);
      bdrv_append(mirror_top_bs, bs, &local_err);
-    bdrv_drained_end(bs);
  
      if (local_err) {
          bdrv_unref(mirror_top_bs);
          error_propagate(errp, local_err);
+        bdrv_drained_end(bs);
          return NULL;
      }
  
@@ -1789,6 +1789,8 @@ static BlockJob *mirror_start_job(
      trace_mirror_start(bs, s, opaque);
      job_start(&s->common.job);
  
+    bdrv_drained_end(bs);
+
      return &s->common;
  
  fail:
@@ -1813,6 +1815,8 @@ fail:
  
      bdrv_unref(mirror_top_bs);
  
+    bdrv_drained_end(bs);
+
      return NULL;
  }
  


Could you check, does it help?


> 
> The rootcause is that qemu 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.
> 
> Signed-off-by: Michael Qiu <qiudayu@huayun.com>
> ---

[*] here you could add any comments, which will not go into final commit 
message, like version history.

>   blockjob.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/blockjob.c b/blockjob.c
> index 98ac8af982..51a09f3b60 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -214,13 +214,15 @@ int block_job_add_bdrv(BlockJob *job, const char *name, 
> BlockDriverState *bs,
>       BdrvChild *c;
>   
>       bdrv_ref(bs);
> -    if (job->job.aio_context != qemu_get_aio_context()) {
> +    if (bdrv_get_aio_context(bs) != job->job.aio_context &&
> +        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 (bdrv_get_aio_context(bs) != job->job.aio_context &&
> +        job->job.aio_context != qemu_get_aio_context()) {

that's a wrong check, it will never reacquire the lock on success path, as 
after successful attach, bs context would definitely equal to job context.

I think you need a boolean variable at start of function, initialized to the 
condition, and after _attach_child() you not recheck the condition but rely on 
variable.

>           aio_context_acquire(job->job.aio_context);
>       }
>       if (c == NULL) {
> 

The code was introduced by Kevin in 132ada80c4a6 "block: Adjust AioContexts 
when attaching nodes", so I think we need his opinion.
You also may add "Fixes: 132ada80c4a6fea7b67e8bb0a5fd299994d927c6", especially 
if you check that your case doesn't fail before this commit.

I think the idea itself is correct, as bdrv_root_attach_child will not call any 
of *_set_aio_*, and no reason to release the lock. So it shouldn't hurt and 
it's great if it fixes some crash.

When side effect of a function is temporary releasing some lock, it's hard to 
be sure that all callers are OK with it...


--
Best regards,
Vladimir