linux-kernel - RCU condition checks

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Date:	Wed, 07 Apr 2010 17:35:30 +0100
From:	David Howells <dhowells@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	dhowells@...hat.com, Eric Dumazet <eric.dumazet@...il.com>,
	Trond.Myklebust@...app.com, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: RCU condition checks

Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> > Why is there a need for 'c'?
> 
> An example use is where rcu_access_pointer() is legal because we are
> either initializing or cleaning up, so that no other CPU has access
> to the pointer.  In these cases, you might do something like:
> 
> 	q = rcu_access_pointer(p->a, p->refcnt == 0);

I think the main problem I have with this is that the fact that p->refcnt
should be 0 here is unrelated to the fact that we're wanting to look at the
value of p->a.  I'd say that this should be two separate statements, for
example:

	ASSERT(p->refcnt == 0);
	q = rcu_access_pointer(p->a);

I could see using a lockdep-managed ASSERT here would work, though.

The other problem I have with this is that I'm assuming rcu_access_pointer()
is simply for looking at the value of the pointer without dereferencing it -
in which case, is there any need for the lock-describing condition?

I agree, though, that:

	q = rcu_dereference_check(p->a,
				  rcu_read_lock_held() || (
				   lockdep_is_held(p->lock) &&
				   lockdep_is_held(q->lock)));

is a reasonable way of keeping the dereference and the lock checks together,
though that could equally well be written, say:

	LOCKDEP_ASSERT(rcu_read_lock_held() || (
		        lockdep_is_held(p->lock) &&
			lockdep_is_held(q->lock)));
	q = rcu_dereference_protected(p->a);

but combining those makes it easier to ensure people to write lock checking.

Davod
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