Fix the memory barrier documentation to attempt to describe atomic ops
correctly.
atomic_t ops that return a value _do_ imply smp_mb() either side, and so don't
actually require smp_mb__*_atomic_*() special barriers.
Also explains why special barriers exist in addition to normal barriers.
Signed-Off-By: David Howells <[email protected]>
---
warthog>diffstat -p1 /tmp/mb.diff
Documentation/memory-barriers.txt | 47 +++++++++++++++++++++++++-------------
1 file changed, 31 insertions(+), 16 deletions(-)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index f855031..822fc45 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -829,8 +829,8 @@ There are some more advanced barrier fun
(*) smp_mb__after_atomic_inc();
These are for use with atomic add, subtract, increment and decrement
- functions, especially when used for reference counting. These functions
- do not imply memory barriers.
+ functions that don't return a value, especially when used for reference
+ counting. These functions do not imply memory barriers.
As an example, consider a piece of code that marks an object as being dead
and then decrements the object's reference count:
@@ -1263,15 +1263,15 @@ else.
ATOMIC OPERATIONS
-----------------
-Though they are technically interprocessor interaction considerations, atomic
-operations are noted specially as they do _not_ generally imply memory
-barriers. The possible offenders include:
+Whilst they are technically interprocessor interaction considerations, atomic
+operations are noted specially as some of them imply full memory barriers and
+some don't, but they're very heavily relied on as a group throughout the
+kernel.
+
+Any atomic_t operation, for instance, that returns a value implies an
+SMP-conditional general memory barrier (smp_mb()) on each side of the actual
+operation. These include:
- xchg();
- cmpxchg();
- test_and_set_bit();
- test_and_clear_bit();
- test_and_change_bit();
atomic_cmpxchg();
atomic_inc_return();
atomic_dec_return();
@@ -1283,20 +1283,30 @@ barriers. The possible offenders includ
atomic_add_negative();
atomic_add_unless();
-These may be used for such things as implementing LOCK operations or controlling
-the lifetime of objects by decreasing their reference counts. In such cases
-they need preceding memory barriers.
-The following may also be possible offenders as they may be used as UNLOCK
-operations.
+The following, however, do _not_ imply memory barrier effects:
+
+ xchg();
+ cmpxchg();
+ test_and_set_bit();
+ test_and_clear_bit();
+ test_and_change_bit();
+
+These may be used for such things as implementing LOCK-class operations. In
+such cases they need explicit memory barriers.
+
+The following are also potential offenders as they may be used as UNLOCK-class
+operations, amongst other things, but do _not_ imply memory barriers either:
set_bit();
clear_bit();
change_bit();
atomic_set();
+With these the appropriate explicit memory barrier should be used if necessary.
+
-The following are a little tricky:
+The following also don't imply memory barriers, and so may be a little tricky:
atomic_add();
atomic_sub();
@@ -1322,6 +1332,11 @@ operation is protected by a lock, then i
there's another operation within the critical section with respect to which an
ordering must be maintained.
+[!] Note that special memory barrier primitives are available for these
+situations because on some CPUs the atomic instructions used imply full memory
+barriers, and so barrier instructions are superfluous in conjunction with them,
+and in such cases the special barrier primitives will be no-ops.
+
See Documentation/atomic_ops.txt for more information.
-
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