On Thu, Jun 07, 2018 at 12:01:57PM +0200, Andrea Parri wrote:
> 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>
Queued for review, thank you!
Thanx, Paul
> 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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