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
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
------------------
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();