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

[Paolo Bonzini]

On 27/06/2017 11:07, Vladimir Sementsov-Ogievskiy wrote:
> 26.06.2017 19:54, Paolo Bonzini wrote:
>>
>> On 26/06/2017 18:07, Vladimir Sementsov-Ogievskiy wrote:
>>> HI!
>>>
>>> One question here, should not 'bdrv_undo_clear_dirty_bitmap' be under
>>> lock too?
>> Any call to dirty bitmap functions between bdrv_clear_dirty_bitmap and
>> bdrv_undo_clear_dirty_bitmap is problematic anyway, so
>> bdrv_clear_dirty_bitmap really only needs the lock in the !out case;
>> bdrv_undo_clear_dirty_bitmap is only called when out != NULL.
>>
>> However, I agree it would be cleaner to add the lock there, too.
> 
> Also, you've added comment "Called with BQL taken" both to functions
> that calls bdrv_dirty_bitmaps_lock and that do not... What is BQL?

It's the "big QEMU lock", also known as "iothread lock".  The locking policy
is documented in block_int.h:

    /* Writing to the list requires the BQL _and_ the dirty_bitmap_mutex.
     * Reading from the list can be done with either the BQL or the
     * dirty_bitmap_mutex.  Modifying a bitmap only requires
     * dirty_bitmap_mutex.  */
    QemuMutex dirty_bitmap_mutex;
    QLIST_HEAD(, BdrvDirtyBitmap) dirty_bitmaps;

and the comments in block/dirty-bitmap.c reflect the above comment.

Paolo