Re: [Qemu-block] [PATCH v5 07/10] qcow2: qcow2_co_preadv: improve locking

[Vladimir Sementsov-Ogievskiy]

30.04.2019 11:38, Vladimir Sementsov-Ogievskiy wrote:
> 29.04.2019 19:37, Max Reitz wrote:
>> On 02.04.19 17:37, Vladimir Sementsov-Ogievskiy wrote:
>>> Background: decryption will be done in threads, to take benefit of it,
>>> we should move it out of the lock first.
>>
>> ...which is safe after your commit c972fa123c73501b4, I presume.
>>
>> (At first glance, the patched looked a bit weird to me because it
>> doesn't give a reason why dropping the lock around
>> qcrypto_block_decrypt() would be OK.)
>>
>>> But let's go further: it turns out, that for locking around switch
>>> cases we have only two variants: when we just do memset(0) not
>>> releasing the lock (it is useless) and when we actually can handle the
>>> whole case out of the lock. So, refactor the whole thing to reduce
>>> locked code region and make it clean.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>> Reviewed-by: Alberto Garcia <address@hidden>
>>> ---
>>>   block/qcow2.c | 46 ++++++++++++++++++++++------------------------
>>>   1 file changed, 22 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/block/qcow2.c b/block/qcow2.c
>>> index 46e8e39da5..fcf92a7eb6 100644
>>> --- a/block/qcow2.c
>>> +++ b/block/qcow2.c
>>> @@ -1983,6 +1983,7 @@ static coroutine_fn int 
>>> qcow2_co_preadv(BlockDriverState *bs, uint64_t offset,
>>>           ret = qcow2_get_cluster_offset(bs, offset, &cur_bytes, 
>>> &cluster_offset);
>>
>> Isn't this the only function in the loop that actually needs the lock?
>> Wouldn't it make more sense to just take it around this call?
>>
> 
> Hmm, looks correct, I'll resend.
> 
> 

Or not, actually, we may have several qcow2_get_data_offset calls under one 
lock,
if clusters are different kinds of ZERO. So, I think better to keep it as is 
for now.



-- 
Best regards,
Vladimir