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

[Paolo Bonzini <pbonzini@redhat.com>]

On 18/01/2018 10:55, Vladimir Sementsov-Ogievskiy wrote:
>>
>> Most functions that looks at the list are "called with BQL taken".
>> Functions that write to the list are "called with BQL taken" and call
>> bdrv_dirty_bitmaps_lock/bdrv_dirty_bitmaps_unlock themselves.
> 
> Paolo, could you please explain about bitmap locking in more details?
> Why do we need mutexes?

We have three cases:

1) monitor creates and destroy bitmaps.

2) monitor also has to read the list.  We know it operates with BQL.

3) users such as mirror.c create a dirty bitmap in the monitor command
(under BQL), but they can operate without BQL in a separate iothread so
we create a separate lock (bitmap->mutex).

While in the second and third case, bitmaps cannot disappear.  So in the
first case you operate with BQL+dirty bitmap mutex.  The result is that
you lock out both the second and the third case while creating and
destroying bitmaps.

> Why do we do not need them
> on read from the bitmap, only on write?

Indeed, reading the bitmap also requires taking the lock.  So
s/Modifying/Accessing/ in that comment.

Paolo