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-rw-r--r--Documentation/memory-barriers.txt46
1 files changed, 13 insertions, 33 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 556f951f8626..f1dc4a215593 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -115,8 +115,8 @@ For example, consider the following sequence of events:
CPU 1 CPU 2
=============== ===============
{ A == 1; B == 2 }
- A = 3; x = A;
- B = 4; y = B;
+ A = 3; x = B;
+ B = 4; y = A;
The set of accesses as seen by the memory system in the middle can be arranged
in 24 different combinations:
@@ -1583,20 +1583,21 @@ There are some more advanced barrier functions:
insert anything more than a compiler barrier in a UP compilation.
- (*) smp_mb__before_atomic_dec();
- (*) smp_mb__after_atomic_dec();
- (*) smp_mb__before_atomic_inc();
- (*) smp_mb__after_atomic_inc();
+ (*) smp_mb__before_atomic();
+ (*) smp_mb__after_atomic();
- These are for use with atomic add, subtract, increment and decrement
- functions that don't return a value, especially when used for reference
- counting. These functions do not imply memory barriers.
+ These are for use with atomic (such as add, subtract, increment and
+ decrement) functions that don't return a value, especially when used for
+ reference counting. These functions do not imply memory barriers.
+
+ These are also used for atomic bitop functions that do not return a
+ value (such as set_bit and clear_bit).
As an example, consider a piece of code that marks an object as being dead
and then decrements the object's reference count:
obj->dead = 1;
- smp_mb__before_atomic_dec();
+ smp_mb__before_atomic();
atomic_dec(&obj->ref_count);
This makes sure that the death mark on the object is perceived to be set
@@ -1606,27 +1607,6 @@ There are some more advanced barrier functions:
operations" subsection for information on where to use these.
- (*) smp_mb__before_clear_bit(void);
- (*) smp_mb__after_clear_bit(void);
-
- These are for use similar to the atomic inc/dec barriers. These are
- typically used for bitwise unlocking operations, so care must be taken as
- there are no implicit memory barriers here either.
-
- Consider implementing an unlock operation of some nature by clearing a
- locking bit. The clear_bit() would then need to be barriered like this:
-
- smp_mb__before_clear_bit();
- clear_bit( ... );
-
- This prevents memory operations before the clear leaking to after it. See
- the subsection on "Locking Functions" with reference to RELEASE operation
- implications.
-
- See Documentation/atomic_ops.txt for more information. See the "Atomic
- operations" subsection for information on where to use these.
-
-
MMIO WRITE BARRIER
------------------
@@ -2283,11 +2263,11 @@ operations:
change_bit();
With these the appropriate explicit memory barrier should be used if necessary
-(smp_mb__before_clear_bit() for instance).
+(smp_mb__before_atomic() for instance).
The following also do _not_ imply memory barriers, and so may require explicit
-memory barriers under some circumstances (smp_mb__before_atomic_dec() for
+memory barriers under some circumstances (smp_mb__before_atomic() for
instance):
atomic_add();