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

[Vladimir Sementsov-Ogievskiy]

24.01.2018 13:16, 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.

yes and query-block actually calls bdrv_query_dirty_bitmaps, which locks 
mutex..

> > 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.

I don't understand. But "Accessing a bitmap only requires 
dirty_bitmap_mutex".. So, if we have several operations on one bitmap,
each of them will lock/unlock mutex by itself? Then we'll have unlocked 
"holes" in our transaction. Or this doesn't matter?

> Paolo


--
Best regards,
Vladimir