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SubjectRe: [PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task_cpu()
On Wed, Jan 16, 2019 at 07:42:18PM +0100, Andrea Parri wrote:
> 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.

In the light of the recent discussion about the integration of plain
accesses in the LKMM (c.f., e.g., [1] and discussion thereof), I was
considering even further changes to this in order to "reinforce" the
above smp_wmb(). Here's two approaches (one of):

1) replace this smp_wmb()+WRITE_ONCE() with an smp_store_release();

2) or keep this smp_wmb()+WRITE_ONCE(), but use {WRITE,READ}_ONCE()
also for the accesses to ->on_rq.

What do you think? (maybe I'm just being too paranoid?)

Adding Will to the Cc: ((1) should be "painless" for x86, not sure
about arm64...)

Andrea

[1] http://lkml.kernel.org/r/20190118155638.GA24442@andrea


>
> 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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