Re: [Qemu-block] [Qemu-devel] [PATCH v9 03/13] block/dirty-bitmap: add _locked version of bdrv_reclaim_dirty_bitmap

[John Snow]

On 01/24/2018 05:16 AM, Paolo Bonzini wrote:
> On 22/01/2018 13:14, Vladimir Sementsov-Ogievskiy wrote:
>>> so, accessing the bitmap needs mutex lock?
>>>
>>> Then what do you mean under accessing the bitmap? Any touch of
>>> BdrvDirtyBitmap fields? Then "reading the list" will require bitmap
>>> mutex too.
>>> Or accessing the bitmap is accessing any field except
>>> BdrvDirtyBitmap.list? Then in (2), what do you mean? For example
>>> query-block will go through
>>> the list, but it touches other fields too, so it should lock mutex.
> 
> The bitmap mutex is internal to block/dirty-bitmap.c.
> 
>> and one more question:
>>
>> What about qmp transactions? Should we lock mutex during the whole
>> transaction?
> 
> Transactions hold the BQL, but you don't need to lock the bitmap mutex.
> 
> Paolo
> 

I would further add that attempting to lock all of the bitmap mutexes
during a transaction might cause complications if you try to perform two
actions on the same bitmap, and it deadlocks.

With the BQL held we're probably okay, but worst-case scenario locking
and unlocking per-each action is probably sufficient, I'd think.