[PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task_cpu()

[PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task_cpu()

[Andrea Parri]

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

Re: [PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task_cpu()

[Andrea Parri]

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
> 

Re: [PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task_cpu()

[Peter Zijlstra]

On Mon, Jan 21, 2019 at 11:51:21AM +0100, Andrea Parri wrote:
> 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.

That should be the least painful I think. Note that we never store a
value larger than a single byte in that word, so tearing shouldn't be a
problem, but yes, that makes it all neat and tidy.

> 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...)

ARM64 should be fine, it is 32bit ARM that will suffer, because it uses
smp_mb() to implement acquire/release.

Re: [PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task_cpu()

[Andrea Parri]

On Mon, Jan 21, 2019 at 01:25:26PM +0100, Peter Zijlstra wrote:
> On Mon, Jan 21, 2019 at 11:51:21AM +0100, Andrea Parri wrote:
> > 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.
> 
> That should be the least painful I think. Note that we never store a
> value larger than a single byte in that word, so tearing shouldn't be a
> problem, but yes, that makes it all neat and tidy.

Thank you for the suggestion; I'll send the revisited patch shortly.

  Andrea