Messages in this thread Patch in this message | | | From | "Paul E. McKenney" <> | Subject | [PATCH tip/core/rcu 21/27] doc: Update synchronize_rcu() definition in whatisRCU.txt | Date | Mon, 25 Jun 2018 17:35:07 -0700 |
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From: Andrea Parri <andrea.parri@amarulasolutions.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@linux.vnet.ibm.com> Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Josh Triplett <josh@joshtriplett.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Cc: Lai Jiangshan <jiangshanlai@gmail.com> Cc: Jonathan Corbet <corbet@lwn.net> Signed-off-by: Paul E. McKenney <paulmck@linux.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
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