Re: [Qemu-devel] [PATCH 3/5] block: convert bdrv_invalidate_cache callback to coroutine_fn

[Paolo Bonzini]

On 11/07/2017 10:24, Kevin Wolf wrote:
> Am 10.07.2017 um 18:58 hat Paolo Bonzini geschrieben:
>> QED's bdrv_invalidate_cache implementation would like to reuse functions
>> that acquire/release the metadata locks.  Call it from coroutine context
>> to simplify the logic.
>>
>> Signed-off-by: Paolo Bonzini <address@hidden>
> 
>> diff --git a/block/qcow2.c b/block/qcow2.c
>> index b5de67d113..4ff7e89009 100644
>> --- a/block/qcow2.c
>> +++ b/block/qcow2.c
>> @@ -1854,7 +1854,8 @@ static void qcow2_close(BlockDriverState *bs)
>>      qcow2_free_snapshots(bs);
>>  }
>>  
>> -static void qcow2_invalidate_cache(BlockDriverState *bs, Error **errp)
>> +static void coroutine_fn qcow2_co_invalidate_cache(BlockDriverState *bs,
>> +                                                   Error **errp)
>>  {
>>      BDRVQcow2State *s = bs->opaque;
>>      int flags = s->flags;
>> @@ -1877,7 +1878,9 @@ static void qcow2_invalidate_cache(BlockDriverState 
>> *bs, Error **errp)
>>      options = qdict_clone_shallow(bs->options);
>>  
>>      flags &= ~BDRV_O_INACTIVE;
>> +    qemu_co_mutex_lock(&s->lock);
>>      ret = qcow2_do_open(bs, options, flags, &local_err);
>> +    qemu_co_mutex_unlock(&s->lock);
>>      QDECREF(options);
>>      if (local_err) {
>>          error_propagate(errp, local_err);
> 
> How useful is this lock really? If we expect any requests while this
> function is running (we don't, it would break horribly), it should
> probably cover qcow2_close(), too.

In the case of QED, this was needed because qed_read_table drops
s->table_lock.  Here I just copied the idiom.

However, I think it's cleaner to define which functions are called with
which lock held.  Sometimes it leads to taking a lock unnecessarily, but
it's clearer and it can also help once we annotate the source and let
the compiler do static analysis (see clang's -Wthread-safety).

Paolo

> Or maybe the cleaner way would be bdrv_drained_begin/end() around
> everything directly in bdrv_co_invalidate_cache().
> 
> Kevin
>