Re: [Qemu-devel] [Qemu-block] [PATCH 1/3] blockjob: fix dead pointer in txn list

[John Snow <jsnow@redhat.com>]

On 08/02/2016 07:05 AM, Vladimir Sementsov-Ogievskiy wrote:
> On 02.08.2016 01:39, John Snow wrote:
>> On 07/27/2016 06:49 AM, Vladimir Sementsov-Ogievskiy wrote:
>>> Job may be freed in block_job_unref and in this case this would break
>>> transaction QLIST.
>>>
>>> Fix this by removing job from this list before unref.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>  blockjob.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/blockjob.c b/blockjob.c
>>> index a5ba3be..e045091 100644
>>> --- a/blockjob.c
>>> +++ b/blockjob.c
>>> @@ -216,6 +216,7 @@ static void block_job_completed_single(BlockJob
>>> *job)
>>>      }
>>>      job->cb(job->opaque, job->ret);
>>>      if (job->txn) {
>>> +        QLIST_REMOVE(job, txn_list);
>>>          block_job_txn_unref(job->txn);
>>>      }
>>>      block_job_unref(job);
>>>
>>
>> Has this caused actual problems for you?
>
> Yes, with the same changed test 124 (my parallel thread). Backup job can
> finish  too early (if dirty bitmap is empty) and then we use this
> transaction job list with dead pointer.
>
>>
>> This function is only ever called in a transactional context if the
>> transaction is over -- so we're not likely to use the pointers ever
>> again anyway.
>
> Backup job may finish even earlier than all jobs are added to the list.
> (same case, empty dirty bitmap for one of drives).
>

AHA, I get it now.

I think the right solution will be a general mechanism at the 
transactional level, not backup-specific hacks, but thank you for 
explaining this to me.

>>
>> Still, it's good practice, and the caller uses a safe iteration of the
>> list, so I think this should be safe.
>>
>> But I don't think this SHOULD fix an actual bug. If it does, I think
>> something else is wrong.
>>
>> Tested-by: John Snow <jsnow@redhat.com>
>> Reviewed-by: John Snow <jsnow@redhat.com>
>
>