linux-kernel - Re: [PATCH] timer: Fix mod_timer_pinned() header commen (was: [PATCH RFC tip/core/rcu 4/4] rcu: Ensure that RCU_FAST_NO

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Date:	Thu, 26 Apr 2012 11:36:51 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH] timer: Fix mod_timer_pinned() header commen (was: [PATCH
 RFC tip/core/rcu 4/4] rcu: Ensure that RCU_FAST_NO_HZ timers expire on
 correct CPU)

On Thu, Apr 26, 2012 at 02:10:19PM -0400, Steven Rostedt wrote:
> 
> > ------------------------------------------------------------------------
> > 
> > timer: Fix mod_timer_pinned() header comment
> > 
> > The mod_timer_pinned() header comment states that it prevents timers
> > from being migrated to a different CPU.  This is not the case, instead,
> > it ensures that the timer is posted to the current CPU, but does nothing
> > to prevent CPU-hotplug operations from migrating the timer.
> > 
> > This commit therefore brings the comment header into alignment with
> > reality.
> > 
> > Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> I'm seeing double :-)

"Bartender, give me a double!"

> > diff --git a/kernel/timer.c b/kernel/timer.c
> > index a297ffc..7114336 100644
> > --- a/kernel/timer.c
> > +++ b/kernel/timer.c
> > @@ -861,7 +861,12 @@ EXPORT_SYMBOL(mod_timer);
> >   *
> >   * mod_timer_pinned() is a way to update the expire field of an
> >   * active timer (if the timer is inactive it will be activated)
> > - * and not allow the timer to be migrated to a different CPU.
> > + * and to ensure that the timer is scheduled on the current CPU.
> 
> Add space here.
> 
>   + *

Good point, done!

> > + * Note that this does not prevent the timer from being migrated
> > + * when the current CPU goes offline.  If this is a problem for
> > + * you, use CPU-hotplug notifiers to handle it correctly, for
> > + * example, cancelling the timer when the corresponding CPU goes
> > + * offline.
> >   *
> >   * mod_timer_pinned(timer, expires) is equivalent to:
> >   *
> 
> Adding a space will make the "Note" stand out more, and more likely seen
> by people who use this function.
> 
> Other than that:
> 
> Acked-by: Steven Rostedt <rostedt@...dmis.org>

Got it, thank you!

							Thanx, Paul

------------------------------------------------------------------------

timer: Fix mod_timer_pinned() header comment

The mod_timer_pinned() header comment states that it prevents timers
from being migrated to a different CPU.  This is not the case, instead,
it ensures that the timer is posted to the current CPU, but does nothing
to prevent CPU-hotplug operations from migrating the timer.

This commit therefore brings the comment header into alignment with
reality.

Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Acked-by: Steven Rostedt <rostedt@...dmis.org>

diff --git a/kernel/timer.c b/kernel/timer.c
index a297ffc..837c552 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -861,7 +861,13 @@ EXPORT_SYMBOL(mod_timer);
  *
  * mod_timer_pinned() is a way to update the expire field of an
  * active timer (if the timer is inactive it will be activated)
- * and not allow the timer to be migrated to a different CPU.
+ * and to ensure that the timer is scheduled on the current CPU.
+ *
+ * Note that this does not prevent the timer from being migrated
+ * when the current CPU goes offline.  If this is a problem for
+ * you, use CPU-hotplug notifiers to handle it correctly, for
+ * example, cancelling the timer when the corresponding CPU goes
+ * offline.
  *
  * mod_timer_pinned(timer, expires) is equivalent to:
  *

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