The synchronize_rcu() definition based on RW-locks in whatisRCU.txt
does not meet the "Memory-Barrier Guarantees" in Requirements.html;
for example, the following SB-like test:
P0: P1:
WRITE_ONCE(x, 1); WRITE_ONCE(y, 1);
synchronize_rcu(); smp_mb();
r0 = READ_ONCE(y); r1 = READ_ONCE(x);
should not be allowed to reach the state "r0 = 0 AND r1 = 0", but
the current write_lock()+write_unlock() definition can not ensure
this. Remedies this by inserting an smp_mb__after_spinlock().
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
---
Documentation/RCU/whatisRCU.txt | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index a27fbfb0efb82..86a54ff911fc2 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -586,6 +586,7 @@ It is extremely simple:
void synchronize_rcu(void)
{
write_lock(&rcu_gp_mutex);
+ smp_mb__after_spinlock();
write_unlock(&rcu_gp_mutex);
}
@@ -607,12 +608,15 @@ don't forget about them when submitting patches making use of RCU!]
The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
and release a global reader-writer lock. The synchronize_rcu()
-primitive write-acquires this same lock, then immediately releases
-it. This means that once synchronize_rcu() exits, all RCU read-side
-critical sections that were in progress before synchronize_rcu() was
-called are guaranteed to have completed -- there is no way that
-synchronize_rcu() would have been able to write-acquire the lock
-otherwise.
+primitive write-acquires this same lock, then releases it. This means
+that once synchronize_rcu() exits, all RCU read-side critical sections
+that were in progress before synchronize_rcu() was called are guaranteed
+to have completed -- there is no way that synchronize_rcu() would have
+been able to write-acquire the lock otherwise. The smp_mb__after_spinlock()
+promotes synchronize_rcu() to a full memory barrier in compliance with
+the "Memory-Barrier Guarantees" listed in:
+
+ Documentation/RCU/Design/Requirements/Requirements.html.
It is possible to nest rcu_read_lock(), since reader-writer locks may
be recursively acquired. Note also that rcu_read_lock() is immune
--
2.7.4
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