Re: [PATCH v5 06/20] jobs: remove aiocontext locks since the functions are under BQL

[Emanuele Giuseppe Esposito]

On 17/02/2022 15:12, Stefan Hajnoczi wrote:
> On Tue, Feb 08, 2022 at 09:34:59AM -0500, Emanuele Giuseppe Esposito wrote:
>> In preparation to the job_lock/unlock patch, remove these
>> aiocontext locks.
>> The main reason these two locks are removed here is because
>> they are inside a loop iterating on the jobs list. Once the
>> job_lock is added, it will have to protect the whole loop,
>> wrapping also the aiocontext acquire/release.
>>
>> We don't want this, as job_lock must be taken inside the AioContext
>> lock, and taking it outside would cause deadlocks.
>>
>> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
>> ---
>>  blockdev.c | 4 ----
>>  job-qmp.c  | 4 ----
>>  2 files changed, 8 deletions(-)
>>
>> diff --git a/blockdev.c b/blockdev.c
>> index 8cac3d739c..e315466914 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -3713,15 +3713,11 @@ BlockJobInfoList *qmp_query_block_jobs(Error **errp)
>>  
>>      for (job = block_job_next(NULL); job; job = block_job_next(job)) {
> 
> I'm confused. block_job_next() is supposed to be called with job_mutex
> held since it iterates the jobs list.

Ok here it is clear that the lock is taken to protect the list. So I
should squash this patch with the one that enables job lock/unlock
(patch 19).

Emanuele

> 
> The patch series might fix this later on but it's hard to review patches
> with broken invariants.
> 
> Does this mean git-bisect(1) is broken since intermediate commits are
> not thread-safe?
> 
>>          BlockJobInfo *value;
>> -        AioContext *aio_context;
>>  
>>          if (block_job_is_internal(job)) {
>>              continue;
>>          }
>> -        aio_context = block_job_get_aio_context(job);
>> -        aio_context_acquire(aio_context);
>>          value = block_job_query(job, errp);
> 
> This function accesses fields that are protected by job_mutex, which we
> don't hold.
>