Subject: Re: [PULL 2/2] vhost: replace rcu with mutex

From: Paolo Bonzini <[email-masked]>
Date: Tue, 03 Jun 2014 15:55:45 +0200

Il 03/06/2014 15:35, Vlad Yasevich ha scritto:
> > Yes, vhost_get_vq_desc must be called with the vq mutex held.
> >
> > The rcu_read_lock/unlock in translate_desc is unnecessary.
>
> If that's true, then does dev->memory really needs to be rcu protected?
> It appears to always be read under mutex.

It's always read under one of many mutexes, yes.

However, it's still RCU-like in the sense that you separate the removal 
and reclamation phases so you still need rcu_dereference/rcu_assign_pointer.

With this mechanism, readers do not contend the mutexes with the 
VHOST_SET_MEMORY ioctl, except for the very short lock-and-unlock 
sequence at the end of it.  They also never contend the mutexes between 
themselves (which would be the case if VHOST_SET_MEMORY locked all the 
mutexes).

You could also wrap all virtqueue processing with a rwsem and take the 
rwsem for write in VHOST_SET_MEMORY.  That simplifies some things however:

- unnecessarily complicates the code for all users of vhost_get_vq_desc

- suppose the reader-writer lock is fair, and VHOST_SET_MEMORY places a 
writer in the queue.  Then a long-running reader R1 could still block 
another reader R2, because the writer would be served before R2.

The RCU-like approach avoids all this, which is important because of the 
generally simpler code and because VHOST_SET_MEMORY is the only vhost 
ioctl that can happen in the hot path.

Paolo