Re: [PATCH v5 tip/core/rcu 14/16] rcu: Remove redundant preempt_disable() from rcu_note_voluntary_context_switch()

[Paul E. McKenney]

On Wed, Aug 13, 2014 at 04:33:10PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 13, 2014 at 07:07:51AM -0700, Paul E. McKenney wrote:
> > On Wed, Aug 13, 2014 at 12:56:18PM +0200, Peter Zijlstra wrote:
> > > On Mon, Aug 11, 2014 at 03:49:03PM -0700, Paul E. McKenney wrote:
> > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > 
> > > > In theory, synchronize_sched() requires a read-side critical section to
> > > > order against.  In practice, preemption can be thought of as being
> > > > disabled across every machine instruction.  So this commit removes
> > > > the redundant preempt_disable() from rcu_note_voluntary_context_switch().
> > > 
> > > >  #define rcu_note_voluntary_context_switch(t) \
> > > >  	do { \
> > > > -		preempt_disable(); /* Exclude synchronize_sched(); */ \
> > > >  		if (ACCESS_ONCE((t)->rcu_tasks_holdout)) \
> > > >  			ACCESS_ONCE((t)->rcu_tasks_holdout) = 0; \
> > > > -		preempt_enable(); \
> > > >  	} while (0)
> > > 
> > > But that's more than 1 instruction.
> > 
> > Yeah, the commit log could use some help.  The instruction in question
> > is the store.  The "if" is just an optimization.
> > 
> > So suppose that this sequence is preempted between the "if" and the store,
> > and that the synchronize_sched() (and quite a bit more besides!) takes
> > place during this preemption.  The task is still in a quiescent state
> > at the time of the store, so the store is still legitimate.
> > 
> > That said, it might be better to just leave preemption disabled, as that
> > certainly makes things simpler.  Thoughts?
> 
> A comment explaining it should be fine I think. I was just raising the
> obvious fail in the changelog.

Fair enough, here is the update.

							Thanx, Paul

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

rcu: Remove redundant preempt_disable() from rcu_note_voluntary_context_switch()

In theory, synchronize_sched() requires a read-side critical section
to order against.  In practice, preemption can be thought of as
being disabled across every machine instruction, at least for those
machine instructions that are not in the idle loop and not on offline
CPUs.  So this commit removes the redundant preempt_disable() from
rcu_note_voluntary_context_switch().

Please note that the single instruction in question is the store of
zero to ->rcu_tasks_holdout.  The "if" is simply a performance optimization
that avoids unnecessary stores.  To see this, keep in mind that both
the "if" condition and the store are in a quiescent state.  Therefore,
even if the task is preempted for a full grace period (presumably due
to its having done a context switch beforehand), the store will be
recording a legitimate quiescent state.

Reported-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index f504f797c9c8..ed6e3e2e0089 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -326,10 +326,8 @@ static inline void rcu_user_hooks_switch(struct task_struct *prev,
 extern struct srcu_struct tasks_rcu_exit_srcu;
 #define rcu_note_voluntary_context_switch(t) \
 	do { \
-		preempt_disable(); /* Exclude synchronize_sched(); */ \
 		if (ACCESS_ONCE((t)->rcu_tasks_holdout)) \
 			ACCESS_ONCE((t)->rcu_tasks_holdout) = 0; \
-		preempt_enable(); \
 	} while (0)
 #else /* #ifdef CONFIG_TASKS_RCU */
 #define TASKS_RCU(x) do { } while (0)

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