rcuwait: Introduce rcuwait_active()

This call is lockless and thus should not be trusted blindly.
For example, the return value of rcuwait_wakeup() should be used to
track whether a process was woken up.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Message-Id: <20200424054837.5138-5-dave@stgolabs.net>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/include/linux/rcuwait.h b/include/linux/rcuwait.h
index 45bc6604e9b1288471e867cb26bdc84673bff63d..c1414ce44abc31c978c220a64286eb144ac9a739 100644 (file)
--- 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);
 
 /*