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

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Message-ID: <538DD9AC.8050606@redhat.com>
Date:	Tue, 03 Jun 2014 16:20:28 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.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

Il 03/06/2014 15:57, Eric Dumazet ha scritto:
> 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

That would make VHOST_SET_MEMORY as slow as before (even though once 
every few times).

>>> 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.

I agree.

Paolo
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