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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 <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>
This commit is contained in:
Andrea Parri 2018-06-07 12:01:57 +02:00 committed by Paul E. McKenney
parent 67abb96cbf
commit 264d4f88ad
1 changed files with 10 additions and 6 deletions
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16
Documentation/RCU/whatisRCU.txt
View File

@ -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