[PATCH urgent v2] sched: Add missing rcu protection to wake_up_all_idle_cpus

[PATCH urgent v2] sched: Add missing rcu protection to wake_up_all_idle_cpus

[Andy Lutomirski]

Locklessly doing is_idle_task(rq->curr) is only okay because of RCU
protection.  The older variant of the broken code checked
rq->curr == rq->idle instead and therefore didn't need RCU.

Fixes: f6be8af1c95d sched: Add new API wake_up_if_idle() to wake up the idle cpu
Cc: Chuansheng Liu <chuansheng.liu@intel.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---

This fixes a 3.18 regression, I think.  This code seems to only be called
inside preempt_disable, so I think it's unlikely to be a problem in practice.

Compile-tested only, although the kbuild bot is chewing on it.

Changes from v1:
 - Cc the right people (whoops)
 - Add rcu_read_lock and rcu_read_unlock.

 kernel/sched/core.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 240157c13ddc..b7937512965f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1623,8 +1623,10 @@ void wake_up_if_idle(int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
 
-	if (!is_idle_task(rq->curr))
-		return;
+	rcu_read_lock();
+
+	if (!is_idle_task(rcu_dereference(rq->curr)))
+		goto out;
 
 	if (set_nr_if_polling(rq->idle)) {
 		trace_sched_wake_idle_without_ipi(cpu);
@@ -1635,6 +1637,9 @@ void wake_up_if_idle(int cpu)
 		/* Else cpu is not in idle, do nothing here */
 		raw_spin_unlock_irqrestore(&rq->lock, flags);
 	}
+
+out:
+	rcu_read_unlock();
 }
 
 bool cpus_share_cache(int this_cpu, int that_cpu)
-- 
1.9.3

RE: [PATCH urgent v2] sched: Add missing rcu protection to wake_up_all_idle_cpus

[Liu, Chuansheng]

> -----Original Message-----
> From: Andy Lutomirski [mailto:luto@amacapital.net]
> Sent: Sunday, November 30, 2014 12:14 AM
> To: Ingo Molnar; Thomas Gleixner; linux-kernel@vger.kernel.org
> Cc: Peter Zijlstra; Andy Lutomirski; Liu, Chuansheng
> Subject: [PATCH urgent v2] sched: Add missing rcu protection to
> wake_up_all_idle_cpus
> 
> Locklessly doing is_idle_task(rq->curr) is only okay because of RCU
> protection.  The older variant of the broken code checked
> rq->curr == rq->idle instead and therefore didn't need RCU.
> 
> Fixes: f6be8af1c95d sched: Add new API wake_up_if_idle() to wake up the idle
> cpu
> Cc: Chuansheng Liu <chuansheng.liu@intel.com>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
Reviewed-by: Chuansheng Liu <chuansheng.liu@intel.com>

Thanks Andy.

[tip:sched/core] sched: Add missing rcu protection to wake_up_all_idle_cpus

[tip-bot for Andy Lutomirski]

Commit-ID:  fd7de1e8d5b2b2b35e71332fafb899f584597150
Gitweb:     http://git.kernel.org/tip/fd7de1e8d5b2b2b35e71332fafb899f584597150
Author:     Andy Lutomirski <luto@amacapital.net>
AuthorDate: Sat, 29 Nov 2014 08:13:51 -0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 8 Dec 2014 11:44:19 +0100

sched: Add missing rcu protection to wake_up_all_idle_cpus

Locklessly doing is_idle_task(rq->curr) is only okay because of
RCU protection.  The older variant of the broken code checked
rq->curr == rq->idle instead and therefore didn't need RCU.

Fixes: f6be8af1c95d ("sched: Add new API wake_up_if_idle() to wake up the idle cpu")
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Chuansheng Liu <chuansheng.liu@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/729365dddca178506dfd0a9451006344cd6808bc.1417277372.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d44d0c5..88f49bc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1628,8 +1628,10 @@ void wake_up_if_idle(int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
 
-	if (!is_idle_task(rq->curr))
-		return;
+	rcu_read_lock();
+
+	if (!is_idle_task(rcu_dereference(rq->curr)))
+		goto out;
 
 	if (set_nr_if_polling(rq->idle)) {
 		trace_sched_wake_idle_without_ipi(cpu);
@@ -1640,6 +1642,9 @@ void wake_up_if_idle(int cpu)
 		/* Else cpu is not in idle, do nothing here */
 		raw_spin_unlock_irqrestore(&rq->lock, flags);
 	}
+
+out:
+	rcu_read_unlock();
 }
 
 bool cpus_share_cache(int this_cpu, int that_cpu)