linux-kernel - [PATCH RT 2/3] posix-timers: move the rcu head out of the union

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Date:   Fri,  4 May 2018 15:55:15 +0200
From:   Daniel Wagner <wagi@...om.org>
To:     linux-rt-users@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Wagner <daniel.wagner@...mens.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Julia Cartwright <julia@...com>, stable-rt@...r.kernel.org
Subject: [PATCH RT 2/3] posix-timers: move the rcu head out of the union

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

On RT the timer can be preempted while running and therefore we wait
with timer_wait_for_callback() for the timer to complete (instead of
busy looping). The RCU-readlock is held to ensure that this posix timer
is not removed while we wait on it.
If the timer is removed then it invokes call_rcu() with a pointer that
is shared with the hrtimer because it is part of the same union.
In order to avoid any possible side effects I am moving the rcu pointer
out of the union.

Cc: stable-rt@...r.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/linux/posix-timers.h | 2 +-
 kernel/time/posix-timers.c   | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
index 907f3fd191ac..e2e43c61f6a1 100644
--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -92,8 +92,8 @@ struct k_itimer {
 			struct alarm alarmtimer;
 			ktime_t interval;
 		} alarm;
-		struct rcu_head rcu;
 	} it;
+	struct rcu_head rcu;
 };
 
 struct k_clock {
diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 48ca053dae2e..0808e2ae49bf 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -567,7 +567,7 @@ static struct k_itimer * alloc_posix_timer(void)
 
 static void k_itimer_rcu_free(struct rcu_head *head)
 {
-	struct k_itimer *tmr = container_of(head, struct k_itimer, it.rcu);
+	struct k_itimer *tmr = container_of(head, struct k_itimer, rcu);
 
 	kmem_cache_free(posix_timers_cache, tmr);
 }
@@ -584,7 +584,7 @@ static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
 	}
 	put_pid(tmr->it_pid);
 	sigqueue_free(tmr->sigq);
-	call_rcu(&tmr->it.rcu, k_itimer_rcu_free);
+	call_rcu(&tmr->rcu, k_itimer_rcu_free);
 }
 
 static struct k_clock *clockid_to_kclock(const clockid_t id)
-- 
2.14.3