Re: [Qemu-devel] [PATCH 16/19] block: protect modification of dirty bitmaps with a mutex

[Paolo Bonzini]

On 27/06/2017 16:43, Vladimir Sementsov-Ogievskiy wrote:
>> The write side is invoked under the 'big QEMU lock' so there cannot be
>> two concurrent writes.
>>
>> A bitmap can be written to after bdrv_find_dirty_bitmap returns, but
>> only if _you_ tell another thread about the bitmap you've just created.
> 
> no, I'm not about touching just created bitmap. I'm about creating
> bitmap with the same name by other thread (unlikely case, but possible).

You can't unless you drop the BQL.

>> If that doesn't happen, the bitmap cannot change.  And it can also
>> disappear because _your_ thread is the one with the big QEMU lock.
> 
> So, if I under BQL, I don't need dirty_bitmap_lock?

Writing to the list requires _both_ BQL and dirty_bitmap_lock.  Write
functions actually have "called with BQL taken" in dirty-bitmap.c,
because dirty-bitmap.c will call dirty_bitmap_lock/unlock itself.

Reading from the list requires one of the two locks.  Such functions
have "called with BQL or dirty_bitmap lock taken".

For reading/writing to the bitmap itself, you need dirty_bitmap_lock.
dirty-bitmap.c can take the lock itself but, there are also functions
named *_locked where the caller takes the lock.

Paolo