linux-kernel - Re: [PATCH] perf: perf_event_exit_task_context: s/rcu_dereference/rcu_dereference

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Date:	Fri, 21 Jan 2011 13:50:16 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, mingo@...hat.com, hpa@...or.com,
	acme@...hat.com, paulus@...ba.org, linux-kernel@...r.kernel.org,
	stern@...land.harvard.edu, a.p.zijlstra@...llo.nl,
	fweisbec@...il.com, roland@...hat.com, tglx@...utronix.de,
	prasad@...ux.vnet.ibm.com, linux-tip-commits@...r.kernel.org
Subject: Re: [PATCH] perf: perf_event_exit_task_context:
 s/rcu_dereference/rcu_dereference_raw/

On Fri, Jan 21, 2011 at 06:53:45PM +0100, Oleg Nesterov wrote:
> On 01/21, Oleg Nesterov wrote:
> >
> > In theory, almost every user of task->child->perf_event_ctxp[]
> > is wrong. find_get_context() can install the new context at any
> > moment, we need read_barrier_depends().
> 
> And perhaps it makes sense to fix them all, although the problem
> is only theoretical.
> 
> > dbe08d82ce3967ccdf459f7951d02589cf967300 "perf: Fix
> > find_get_context() vs perf_event_exit_task() race" added
> > rcu_dereference() into perf_event_exit_task_context() to make
> > the precedent, but this makes __rcu_dereference_check() unhappy.
> > Use rcu_dereference_raw() to shut up the warning.
> 
> But rcu_dereference_raw() looks a bit confusing, and it is not
> very convenient to use read_barrier_depends() directly.
> 
> Paul, may be it makes sense to add the new trivial helper which
> can be used instead?
> 
> Yes, this is only cosmetic issue, I know ;)

Cosmetic issues can be pretty important to the poor guy trying to read
the code.  ;-)

What keeps the structure that rcu_dereference_raw() returns a pointer
to from going away?  Best would be if a lockdep condition could be
constructed from the answer to this question and added to the appropriate
rcu_dereference() primitive.

							Thanx, Paul
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