linux-kernel - [PATCH 4/5] rcuwait: Introduce rcuwait

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Date:   Thu, 23 Apr 2020 22:48:36 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     tglx@...utronix.de, pbonzini@...hat.com
Cc:     peterz@...radead.org, maz@...nel.org, bigeasy@...utronix.de,
        rostedt@...dmis.org, torvalds@...ux-foundation.org,
        will@...nel.org, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        dave@...olabs.net, Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 4/5] rcuwait: Introduce rcuwait_active()

This call is lockless and thus should not be trustedblindly,
ie: for wakeup purposes, which is already provided correctly
by rcuwait_wakeup().

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 include/linux/rcuwait.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/linux/rcuwait.h b/include/linux/rcuwait.h
index 45bc6604e9b1..c1414ce44abc 100644
--- a/include/linux/rcuwait.h
+++ b/include/linux/rcuwait.h
@@ -25,6 +25,15 @@ static inline void rcuwait_init(struct rcuwait *w)
 	w->task = NULL;
 }
 
+/*
+ * Note: this provides no serialization and, just as with waitqueues,
+ * requires care to estimate as to whether or not the wait is active.
+ */
+static inline int rcuwait_active(struct rcuwait *w)
+{
+	return !!rcu_dereference(w->task);
+}
+
 extern int rcuwait_wake_up(struct rcuwait *w);
 
 /*
-- 
2.16.4