linux-kernel - Re: Question on comment header for for_each

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Message-Id: <20181108153109.GF4170@linux.ibm.com>
Date:   Thu, 8 Nov 2018 07:31:09 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: Question on comment header for for_each_domain()

On Thu, Nov 08, 2018 at 10:21:51AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 07, 2018 at 03:00:02PM -0800, Paul E. McKenney wrote:
> > Hello!
> > 
> > The header comment for for_each_domain() talks about a call to
> > synchronize_sched() within detach_destroy_domains(), but I am not
> > seeing any such call.  Because synchronize_sched() is now folded into
> > synchronize_rcu(), I have a patch that edits the comment, but it looks
> > like a larger change is needed.
> > 
> > Or am I blind today?
> 
> I think you're quite right and that comment is a wee bit stale.
> 
> The sched domain tree is indeed protected by regular RCU (not RCU-sched
> as the comment seems to imply) and this is per destroy_sched_domains()
> using call_rcu().
> 
> And most (I didn't look at all) uses for the sched-domain tree do indeed
> employ rcu_read_lock().

Ah, thank you for the info!  Would this patch do the trick?

							Thanx, Paul

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

commit 4182d416309b11d16e882ab637ab11cecef0bddc
Author: Paul E. McKenney <paulmck@...ux.ibm.com>
Date:   Tue Nov 6 19:10:53 2018 -0800

    sched: Replace call_rcu_sched() with call_rcu()
    
    Now that call_rcu()'s callback is not invoked until after all
    preempt-disable regions of code have completed (in addition to explicitly
    marked RCU read-side critical sections), call_rcu() can be used in place
    of call_rcu_sched().  This commit therefore makes that change.
    
    While in the area, this commit also updates an outdated header comment
    for for_each_domain().
    
    Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
    Cc: Ingo Molnar <mingo@...hat.com>
    Cc: Peter Zijlstra <peterz@...radead.org>

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 618577fc9aa8..00b91d16af9f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1237,7 +1237,7 @@ extern void sched_ttwu_pending(void);
 
 /*
  * The domain tree (rq->sd) is protected by RCU's quiescent state transition.
- * See detach_destroy_domains: synchronize_sched for details.
+ * See destroy_sched_domains: call_rcu for details.
  *
  * The domain tree of any CPU may only be accessed from within
  * preempt-disabled sections.
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 8d7f15ba5916..04d458faf2c1 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -248,7 +248,7 @@ void rq_attach_root(struct rq *rq, struct root_domain *rd)
 	raw_spin_unlock_irqrestore(&rq->lock, flags);
 
 	if (old_rd)
-		call_rcu_sched(&old_rd->rcu, free_rootdomain);
+		call_rcu(&old_rd->rcu, free_rootdomain);
 }
 
 void sched_get_rd(struct root_domain *rd)
@@ -261,7 +261,7 @@ void sched_put_rd(struct root_domain *rd)
 	if (!atomic_dec_and_test(&rd->refcount))
 		return;
 
-	call_rcu_sched(&rd->rcu, free_rootdomain);
+	call_rcu(&rd->rcu, free_rootdomain);
 }
 
 static int init_rootdomain(struct root_domain *rd)