Re: [PATCH 01/12] introduce BDRV_POLL_WHILE_UNLOCKED

[Emanuele Giuseppe Esposito]

On 26/01/2022 11:49, Stefan Hajnoczi wrote:
> On Tue, Jan 18, 2022 at 11:27:27AM -0500, Emanuele Giuseppe Esposito wrote:
>> Same as BDRV_POLL_WHILE, but uses AIO_WAIT_WHILE_UNLOCKED.
>>
>> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
>> ---
>>  include/block/block-global-state.h | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/include/block/block-global-state.h 
>> b/include/block/block-global-state.h
>> index 419fe8427f..7ad9496f56 100644
>> --- a/include/block/block-global-state.h
>> +++ b/include/block/block-global-state.h
>> @@ -158,6 +158,11 @@ void bdrv_drain_all(void);
>>      AIO_WAIT_WHILE(bdrv_get_aio_context(bs_),              \
>>                     cond); })
>>  
>> +#define BDRV_POLL_WHILE_UNLOCKED(bs, cond) ({              \
>> +    BlockDriverState *bs_ = (bs);                          \
>> +    AIO_WAIT_WHILE_UNLOCKED(bdrv_get_aio_context(bs_),     \
>> +                            cond); })
> 
> No doc comments? When and why is this API useful? Are there any
> preconditions or assumptions (e.g. cond must be thread-safe)?
> 
I am planning to add the following comment above the macro:

/*
 * Unlocked counterpart of BDRV_POLL_WHILE. Uses AIO_WAIT_WHILE_UNLOCKED,
 * so it does not release+acquire the AioContext lock if we are in the
 * main loop, therefore the caller does not need to take it.
 * This function avoids taking the AioContext lock unnecessarly, so use
 * it only when sure that the lock is not taken already, otherwise it
 * might cause deadlocks.
 *
 * @cond must be thread-safe.
 */

Emanuele