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

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Date:	Fri, 21 Jan 2011 18:53:45 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	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,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] perf: perf_event_exit_task_context:
	s/rcu_dereference/rcu_dereference_raw/

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 ;)

Oleg.

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