documentation: memory-barriers: Fix smp_mb__before_spinlock() semantics

Our current documentation claims that, when followed by an ACQUIRE,
smp_mb__before_spinlock() orders prior loads against subsequent loads
and stores, which isn't the intent.  This commit therefore fixes the
documentation to state that this sequence orders only prior stores
against subsequent loads and stores.

In addition, the original intent of smp_mb__before_spinlock() was to only
order prior loads against subsequent stores, however, people have started
using it as if it ordered prior loads against subsequent loads and stores.
This commit therefore also updates smp_mb__before_spinlock()'s header
comment to reflect this new reality.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index f95746189b5ded41d9ae8bdbbcafec4e1da8820e..1f362fd2ecb42de3075f8a442b5ab8f0abbf61a1 100644 (file)
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1784,10 +1784,9 @@ for each construct.  These operations all imply certain barriers:
 
      Memory operations issued before the ACQUIRE may be completed after
      the ACQUIRE operation has completed.  An smp_mb__before_spinlock(),
-     combined with a following ACQUIRE, orders prior loads against
-     subsequent loads and stores and also orders prior stores against
-     subsequent stores.  Note that this is weaker than smp_mb()!  The
-     smp_mb__before_spinlock() primitive is free on many architectures.
+     combined with a following ACQUIRE, orders prior stores against
+     subsequent loads and stores. Note that this is weaker than smp_mb()!
+     The smp_mb__before_spinlock() primitive is free on many architectures.
 
  (2) RELEASE operation implication:
 

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 3e18379dfa6f349ba48edfc6615232af41ccfb99..0063b24b4f36df594b3587daadfdaf8849192c7d 100644 (file)
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -120,7 +120,7 @@ do {                                                                \
 /*
  * Despite its name it doesn't necessarily has to be a full barrier.
  * It should only guarantee that a STORE before the critical section
- * can not be reordered with a LOAD inside this section.
+ * can not be reordered with LOADs and STOREs inside this section.
  * spin_lock() is the one-way barrier, this LOAD can not escape out
  * of the region. So the default implementation simply ensures that
  * a STORE can not move into the critical section, smp_wmb() should