linux-kernel - Re: [PATCH RFC tip/core/rcu 2/5] rcu: Short-circuit normal GPs via expedited GPs

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Message-ID: <20150702193545.GQ3717@linux.vnet.ibm.com>
Date:	Thu, 2 Jul 2015 12:35:45 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 2/5] rcu: Short-circuit normal GPs via
 expedited GPs

On Thu, Jul 02, 2015 at 06:48:16PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 02, 2015 at 07:06:17AM -0700, Paul E. McKenney wrote:
> > Or is your point that RCU_GP_DONE_FQS is a bad name?  Perhaps I should
> > change it to something like RCU_GP_DOING_FQS.  Or am I still missing
> > something here?
> 
> Yes, that might have been the root of my confusion.

OK, how about this, then?

							Thanx, Paul

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

rcu: Rename RCU_GP_DONE_FQS to RCU_GP_DOING_FQS

The grace-period kthread sleeps waiting to do a force-quiescent-state
scan, and when awakened sets rsp->gp_state to RCU_GP_DONE_FQS.
However, this is confusing because the kthread has not done the
force-quiescent-state, but is instead just starting to do it.  This commit
therefore renames RCU_GP_DONE_FQS to RCU_GP_DOING_FQS in order to make
things a bit easier on reviewers.

Reported-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2afb8e8c5134..e1d9909fd59d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2088,7 +2088,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 			rsp->gp_state = RCU_GP_WAIT_FQS;
 			ret = wait_event_interruptible_timeout(rsp->gp_wq,
 					rcu_gp_fqs_check_wake(rsp, &gf), j);
-			rsp->gp_state = RCU_GP_DONE_FQS;
+			rsp->gp_state = RCU_GP_DOING_FQS;
 			/* Locking provides needed memory barriers. */
 			/* If grace period done, leave loop. */
 			if (!READ_ONCE(rnp->qsmask) &&
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index cb402b5c2e71..852b810df38e 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -533,7 +533,7 @@ struct rcu_state {
 #define RCU_GP_WAIT_GPS  1	/* Wait for grace-period start. */
 #define RCU_GP_DONE_GPS  2	/* Wait done for grace-period start. */
 #define RCU_GP_WAIT_FQS  3	/* Wait for force-quiescent-state time. */
-#define RCU_GP_DONE_FQS  4	/* Wait done for force-quiescent-state time. */
+#define RCU_GP_DOING_FQS 4	/* Wait done for force-quiescent-state time. */
 #define RCU_GP_CLEANUP   5	/* Grace-period cleanup started. */
 #define RCU_GP_CLEANED   6	/* Grace-period cleanup complete. */
 

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