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-rw-r--r--Documentation/memory-barriers.txt14
1 files changed, 12 insertions, 2 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 650657c..4e17beb 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1479,7 +1479,8 @@ kernel.
Any atomic operation that modifies some state in memory and returns information
about the state (old or new) implies an SMP-conditional general memory barrier
-(smp_mb()) on each side of the actual operation. These include:
+(smp_mb()) on each side of the actual operation (with the exception of
+explicit lock operations, described later). These include:
xchg();
cmpxchg();
@@ -1536,10 +1537,19 @@ If they're used for constructing a lock of some description, then they probably
do need memory barriers as a lock primitive generally has to do things in a
specific order.
-
Basically, each usage case has to be carefully considered as to whether memory
barriers are needed or not.
+The following operations are special locking primitives:
+
+ test_and_set_bit_lock();
+ clear_bit_unlock();
+ __clear_bit_unlock();
+
+These implement LOCK-class and UNLOCK-class operations. These should be used in
+preference to other operations when implementing locking primitives, because
+their implementations can be optimised on many architectures.
+
[!] 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,
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