From 264d4f88ad5ba0d3c890a70a1216b4e87e5c26ec Mon Sep 17 00:00:00 2001 From: Andrea Parri Date: Thu, 7 Jun 2018 12:01:57 +0200 Subject: doc: Update synchronize_rcu() definition in whatisRCU.txt 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 Signed-off-by: Andrea Parri Cc: Paul E. McKenney Cc: Josh Triplett Cc: Steven Rostedt Cc: Mathieu Desnoyers Cc: Lai Jiangshan Cc: Jonathan Corbet Signed-off-by: Paul E. McKenney --- Documentation/RCU/whatisRCU.txt | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) (limited to 'Documentation/RCU/whatisRCU.txt') 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 -- cgit v1.2.3