Re: [Qemu-devel] [PATCH v9 3/8] migration: use bitmap_mutex in migration_bitmap_clear_dirty

[Wei Wang]

On 11/27/2018 01:40 PM, Peter Xu wrote:
> On Thu, Nov 15, 2018 at 06:07:59PM +0800, Wei Wang wrote:
> > The bitmap mutex is used to synchronize threads to update the dirty
> > bitmap and the migration_dirty_pages counter. For example, the free
> > page optimization clears bits of free pages from the bitmap in an
> > iothread context. This patch makes migration_bitmap_clear_dirty update
> > the bitmap and counter under the mutex.
> >
> > Signed-off-by: Wei Wang <address@hidden>
> > CC: Dr. David Alan Gilbert <address@hidden>
> > CC: Juan Quintela <address@hidden>
> > CC: Michael S. Tsirkin <address@hidden>
> > CC: Peter Xu <address@hidden>
> > ---
> >   migration/ram.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/migration/ram.c b/migration/ram.c
> > index 7e7deec..ef69dbe 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -1556,11 +1556,14 @@ static inline bool 
> > migration_bitmap_clear_dirty(RAMState *rs,
> >   {
> >       bool ret;
> >   
> > +    qemu_mutex_lock(&rs->bitmap_mutex);
> >       ret = test_and_clear_bit(page, rb->bmap);
> >   
> >       if (ret) {
> >           rs->migration_dirty_pages--;
> >       }
> > +    qemu_mutex_unlock(&rs->bitmap_mutex);
> > +
> >       return ret;
> >   }
> It seems fine to me, but have you thought about
> test_and_clear_bit_atomic()?  Note that we just had
> test_and_set_bit_atomic() a few months ago.

Thanks for sharing. I think we might also need to
mutex migration_dirty_pages.

> And not related to this patch: I'm unclear on why we have had
> bitmap_mutex before, since it seems unnecessary.

OK. This is because with the optimization we have a thread
which clears bits (of free pages) from the bitmap and updates
migration_dirty_pages. So we need to synchronization between
the migration thread and the optimization thread.

Best,
Wei