diff options
Diffstat (limited to 'Documentation/memory-barriers.txt')
-rw-r--r-- | Documentation/memory-barriers.txt | 46 |
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(); |