linux-kernel - [PATCH tip/core/rcu 01/18] rcu/nocb: Use separate flag to indicate disabled ->cblist

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Message-Id: <20190801230810.21570-1-paulmck@linux.ibm.com>
Date:   Thu,  1 Aug 2019 16:07:53 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 01/18] rcu/nocb: Use separate flag to indicate disabled ->cblist

NULLing the RCU_NEXT_TAIL pointer was a clever way to save a byte, but
forward-progress considerations would require that this pointer be both
NULL and non-NULL, which, absent a quantum-computer port of the Linux
kernel, simply won't happen.  This commit therefore creates as separate
->enabled flag to replace the current NULL checks.

Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 include/linux/rcu_segcblist.h | 1 +
 kernel/rcu/rcu_segcblist.c    | 3 ++-
 kernel/rcu/rcu_segcblist.h    | 2 +-
 kernel/rcu/tree_plugin.h      | 2 +-
 4 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
index 87404cb015f1..f48888040332 100644
--- a/include/linux/rcu_segcblist.h
+++ b/include/linux/rcu_segcblist.h
@@ -67,6 +67,7 @@ struct rcu_segcblist {
 	unsigned long gp_seq[RCU_CBLIST_NSEGS];
 	long len;
 	long len_lazy;
+	u8 enabled;
 };
 
 #define RCU_SEGCBLIST_INITIALIZER(n) \
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 9bd5f6023c21..b305dcac34c9 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -58,6 +58,7 @@ void rcu_segcblist_init(struct rcu_segcblist *rsclp)
 		rsclp->tails[i] = &rsclp->head;
 	rsclp->len = 0;
 	rsclp->len_lazy = 0;
+	rsclp->enabled = 1;
 }
 
 /*
@@ -69,7 +70,7 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
 	WARN_ON_ONCE(!rcu_segcblist_empty(rsclp));
 	WARN_ON_ONCE(rcu_segcblist_n_cbs(rsclp));
 	WARN_ON_ONCE(rcu_segcblist_n_lazy_cbs(rsclp));
-	rsclp->tails[RCU_NEXT_TAIL] = NULL;
+	rsclp->enabled = 0;
 }
 
 /*
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 822a39da0533..b2de7b32da29 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -63,7 +63,7 @@ static inline long rcu_segcblist_n_nonlazy_cbs(struct rcu_segcblist *rsclp)
  */
 static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
 {
-	return !!rsclp->tails[RCU_NEXT_TAIL];
+	return rsclp->enabled;
 }
 
 /*
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0a3f8680b450..b8a43cf9bb4e 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2189,8 +2189,8 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
 				rcu_segcblist_n_cbs(&rdp->cblist));
 		atomic_long_set(&rdp->nocb_q_count_lazy,
 				rcu_segcblist_n_lazy_cbs(&rdp->cblist));
-		rcu_segcblist_init(&rdp->cblist);
 	}
+	rcu_segcblist_init(&rdp->cblist);
 	rcu_segcblist_disable(&rdp->cblist);
 	return true;
 }
-- 
2.17.1