Re: [RFC PATCH v3 3/4] lock to protect memslots

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Aug 12, 2011 at 08:45:50AM +0200, Paolo Bonzini wrote:
> On 08/11/2011 06:20 PM, Paolo Bonzini wrote:
> >>
> >>+                qemu_mutex_lock_ramlist();
> >>                  QLIST_REMOVE(block, next);
> >>                  QLIST_INSERT_HEAD(&ram_list.blocks, block, next);
> >>+                qemu_mutex_unlock_ramlist();
> >
> >Theoretically qemu_get_ram_ptr should be protected.  The problem is not
> >just accessing the ramlist, it is accessing the data underneath it
> >before anyone frees it.  Luckily we can set aside that problem for now,
> >because qemu_ram_free_from_ptr is only used by device assignment and
> >device assignment makes VMs unmigratable.
> 
> Hmm, rethinking about it, all the loops in exec.c should be
> protected from the mutex.  That's not too good because
> qemu_get_ram_ptr is a hot path for TCG.

Right.

>  Perhaps you can also avoid
> the mutex entirely, and just disable the above optimization for
> most-recently-used-block while migration is running.  It's not a
> complete solution, but it could be good enough until we have RAM
> hot-plug/hot-unplug.

Actually the previous patchset does not traverse the ramlist without 
qemu_mutex locked, which is safe versus the most-recently-used-block
optimization.

So, agreed that the new ramlist lock is not necessary for now.