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Subject[PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task_cpu()
Date
The smp_wmb() in move_queued_task() (c.f., __set_task_cpu()) pairs with
the composition of the dependency and the ACQUIRE in task_rq_lock():

move_queued_task() task_rq_lock()

[S] ->on_rq = MIGRATING [L] rq = task_rq()
WMB (__set_task_cpu()) ACQUIRE (rq->lock);
[S] ->cpu = new_cpu [L] ->on_rq

where "[L] rq = task_rq()" is ordered before "ACQUIRE (rq->lock)" by an
address dependency and, in turn, "ACQUIRE (rq->lock)" is ordered before
"[L] ->on_rq" by the ACQUIRE itself.

Use READ_ONCE() to load ->cpu in task_rq() (c.f., task_cpu()) to honour
this address dependency between loads; also, mark the store to ->cpu in
__set_task_cpu() by using WRITE_ONCE() in order to tell the compiler to
not mess/tear this (synchronizing) memory access.

Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
---
include/linux/sched.h | 4 ++--
kernel/sched/sched.h | 4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 224666226e87b..2bb02c9635bd8 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1753,9 +1753,9 @@ static __always_inline bool need_resched(void)
static inline unsigned int task_cpu(const struct task_struct *p)
{
#ifdef CONFIG_THREAD_INFO_IN_TASK
- return p->cpu;
+ return READ_ONCE(p->cpu);
#else
- return task_thread_info(p)->cpu;
+ return READ_ONCE(task_thread_info(p)->cpu);
#endif
}

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d04530bf251fe..270a3333589d2 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1460,9 +1460,9 @@ static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu)
*/
smp_wmb();
#ifdef CONFIG_THREAD_INFO_IN_TASK
- p->cpu = cpu;
+ WRITE_ONCE(p->cpu, cpu);
#else
- task_thread_info(p)->cpu = cpu;
+ WRITE_ONCE(task_thread_info(p)->cpu, cpu);
#endif
p->wake_cpu = cpu;
#endif
--
2.17.1
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