linux-kernel - Re: [PULL 2/2] vhost: replace rcu with mutex

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Message-ID: <1401803863.3645.208.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 03 Jun 2014 06:57:43 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	David Miller <davem@...emloft.net>
Subject: Re: [PULL 2/2] vhost: replace rcu with mutex

On Tue, 2014-06-03 at 14:48 +0200, Paolo Bonzini wrote:
> Il 02/06/2014 23:58, Eric Dumazet ha scritto:
> > This looks dubious
> >
> > What about using kfree_rcu() instead ?
> 
> It would lead to unbound allocation from userspace.

Look at how we did this in commit
c3059477fce2d956a0bb3e04357324780c5d8eeb

> 
> > translate_desc() still uses rcu_read_lock(), its not clear if the mutex
> > is really held.
> 
> Yes, vhost_get_vq_desc must be called with the vq mutex held.
> 
> The rcu_read_lock/unlock in translate_desc is unnecessary.

Yep, this is what I pointed out. This is not only necessary, but
confusing and might be incorrectly copy/pasted in the future.

This patch is a partial one and leaves confusion.

Some places uses the proper 

mp = rcu_dereference_protected(dev->memory,
		 lockdep_is_held(&dev->mutex));

others use the now incorrect :

rcu_read_lock();
mp = rcu_dereference(dev->memory);
...



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