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

[Vladimir Sementsov-Ogievskiy]

27.06.2017 15:52, Paolo Bonzini wrote:
> On 27/06/2017 11:47, Vladimir Sementsov-Ogievskiy wrote:
> > bdrv_enable_dirty_bitmap - writes to the list, it changes 'disabled'
> > field. So it requires both BQL and dirty_bitmap_mutex? But the comment
> > says only about BQL.
> This one is interesting.  There could be a concurrent call to
> bdrv_set_dirty, indeed.  I'll send a patch.
>
> > Also, for example, if I want to create a new bitmap and than somehow
> > change it, should I do it like this:
> >
> > bdrv_create_dirty_bitmap(...)
> >
> > bdrv_dirty_bitmaps_lock(bs)
> >
> > bitmap  = bdrv_find_dirty_bitmap(bs, name)
> >
> > <some changes>
> >
> > bdrv_dirty_bitmaps_unlock(bs)
> >
> > - because we can't now trust the pointer returned by
> > bdrv_create_dirty_bitmap, as it releases bitmap lock before return.
> If you have the big QEMU lock (you do if you are in the QEMU monitor),
> you are protected from changes to the list of bitmaps.
>
> Paolo

but you wrote "Writing to the list requires the BQL _and_ the 
dirty_bitmap_mutex".

should it be BQL only?


--
Best regards,
Vladimir