On Tue, May 13, 2014 at 07:03:20AM +0000, Gonglei (Arei) wrote: > Hi, > > > -----Original Message----- > > From: Michael S. Tsirkin [mailto:m...@redhat.com] > > Sent: Monday, May 12, 2014 6:31 PM > > > > vhost does everything under a VQ lock. > > I think RCU for VHOST_SET_MEM_TABLE can be replaced with > > taking and freeing the VQ lock. > > > > Does the below solve the problem for you > > (warning: untested, sorry, busy with other bugs right now)? > > > > > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > > > > --- > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > index 78987e4..df2e3eb 100644 > > --- a/drivers/vhost/vhost.c > > +++ b/drivers/vhost/vhost.c > > @@ -593,6 +593,7 @@ static long vhost_set_memory(struct vhost_dev *d, > > struct vhost_memory __user *m) > > { > > struct vhost_memory mem, *newmem, *oldmem; > > unsigned long size = offsetof(struct vhost_memory, regions); > > + int i; > > > > if (copy_from_user(&mem, m, size)) > > return -EFAULT; > > @@ -619,7 +620,14 @@ static long vhost_set_memory(struct vhost_dev *d, > > struct vhost_memory __user *m) > > oldmem = rcu_dereference_protected(d->memory, > > lockdep_is_held(&d->mutex)); > > rcu_assign_pointer(d->memory, newmem); > > - synchronize_rcu(); > > + > > + /* All memory accesses are done under some VQ mutex. > > + * So below is a faster equivalent of synchronize_rcu() > > + */ > > + for (i = 0; i < dev->nvqs; ++i) { > > + mutex_lock(d->vqs[idx]->mutex); > > + mutex_unlock(d->vqs[idx]->mutex); > > + } > > kfree(oldmem); > > return 0; > > } > > Thanks for your advice, I suppose getting mutexes should generally be faster > than waiting for > CPU context switches. And I think d->mutex should also be synchronized since > somewhere gets > only this mutex directly and not vq mutexes. Is this right?
No because all memory table accesses are under some vq mutex. > I'll try this approach, thanks. > > Best regards, > -Gonglei
