linux-kernel - [PATCH tip/core/rcu 21/27] doc: Update synchronize

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Date:   Mon, 25 Jun 2018 17:35:07 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     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,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Jonathan Corbet <corbet@....net>
Subject: [PATCH tip/core/rcu 21/27] doc: Update synchronize_rcu() definition in whatisRCU.txt

From: Andrea Parri <andrea.parri@...rulasolutions.com>

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.  This commit therefore inserts an smp_mb__after_spinlock()
in order to cause this synchronize_rcu() implementation to provide
this memory-barrier guarantee.

Suggested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Jonathan Corbet <corbet@....net>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 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 65eb856526b7..94288f1b8759 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -588,6 +588,7 @@ It is extremely simple:
 	void synchronize_rcu(void)
 	{
 		write_lock(&rcu_gp_mutex);
+		smp_mb__after_spinlock();
 		write_unlock(&rcu_gp_mutex);
 	}
 
@@ -609,12 +610,15 @@ 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
-- 
2.17.1