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

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/02/2018 18:30, Vladimir Sementsov-Ogievskiy wrote:
> 18.01.2018 13:09, Paolo Bonzini wrote:>> 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.
> 
> So, finally, the whole thing is:
> 
> 1. any access to dirty_bitmaps list needs BQL or dirty_bitmap_mutex
> 2. bitmap creation or removing needs both BQL and dirty_bitmap_mutex

3. any access to a dirty bitmap needs dirty_bitmap_mutex

Paolo

> yes?
> 
> and one more question:
> Do we really have users, which accesses dirty bitmaps with only BQL?
> query-block uses dirty_bitmap_mutex..
> 
>