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

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Message-ID: <20110121174547.GA8796@redhat.com>
Date:	Fri, 21 Jan 2011 18:45:47 +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: [PATCH] perf: perf_event_exit_task_context:
	s/rcu_dereference/rcu_dereference_raw/

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().

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.

Reported-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---

 kernel/perf_event.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- git/kernel/perf_event.c~6_rcu_check	2011-01-19 18:49:23.000000000 +0100
+++ git/kernel/perf_event.c	2011-01-21 18:41:02.000000000 +0100
@@ -6133,7 +6133,7 @@ static void perf_event_exit_task_context
 	 * scheduled, so we are now safe from rescheduling changing
 	 * our context.
 	 */
-	child_ctx = rcu_dereference(child->perf_event_ctxp[ctxn]);
+	child_ctx = rcu_dereference_raw(child->perf_event_ctxp[ctxn]);
 	task_ctx_sched_out(child_ctx, EVENT_ALL);
 
 	/*

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