linux-kernel - [PATCH 1/3] sched: Remove __rcu annotation from cred pointer

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Message-Id: <20200207180504.4200-1-frextrite@gmail.com>
Date:   Fri,  7 Feb 2020 23:35:03 +0530
From:   Amol Grover <frextrite@...il.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        James Morris <jamorris@...ux.microsoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...hat.com>
Cc:     linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel@...r.kernel.org, linux-audit@...hat.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Amol Grover <frextrite@...il.com>
Subject: [PATCH 1/3] sched: Remove __rcu annotation from cred pointer

task_struct::cred (subjective credentials) is *always* used
task-synchronously, hence, does not require RCU semantics.

task_struct::real_cred (objective credentials) can be used in
RCU context and its __rcu annotation is retained.

However, task_struct::cred and task_struct::real_cred *may*
point to the same object, hence, the object pointed to by
task_struct::cred *may* have RCU delayed freeing.

Suggested-by: Jann Horn <jannh@...gle.com>
Co-developed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Amol Grover <frextrite@...il.com>
---
 include/linux/sched.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 716ad1d8d95e..39924e6e0cf2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -879,8 +879,11 @@ struct task_struct {
 	/* Objective and real subjective task credentials (COW): */
 	const struct cred __rcu		*real_cred;
 
-	/* Effective (overridable) subjective task credentials (COW): */
-	const struct cred __rcu		*cred;
+	/*
+	 * Effective (overridable) subjective task credentials (COW)
+	 * which is used task-synchronously
+	 */
+	const struct cred		*cred;
 
 #ifdef CONFIG_KEYS
 	/* Cached requested key. */
-- 
2.24.1