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-rw-r--r--Documentation/memory-barriers.txt27
1 files changed, 23 insertions, 4 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index f1dc4a2..a4de88f 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -757,10 +757,14 @@ SMP BARRIER PAIRING
When dealing with CPU-CPU interactions, certain types of memory barrier should
always be paired. A lack of appropriate pairing is almost certainly an error.
-A write barrier should always be paired with a data dependency barrier or read
-barrier, though a general barrier would also be viable. Similarly a read
-barrier or a data dependency barrier should always be paired with at least an
-write barrier, though, again, a general barrier is viable:
+General barriers pair with each other, though they also pair with
+most other types of barriers, albeit without transitivity. An acquire
+barrier pairs with a release barrier, but both may also pair with other
+barriers, including of course general barriers. A write barrier pairs
+with a data dependency barrier, an acquire barrier, a release barrier,
+a read barrier, or a general barrier. Similarly a read barrier or a
+data dependency barrier pairs with a write barrier, an acquire barrier,
+a release barrier, or a general barrier:
CPU 1 CPU 2
=============== ===============
@@ -1893,6 +1897,21 @@ between the STORE to indicate the event and the STORE to set TASK_RUNNING:
<general barrier> STORE current->state
LOAD event_indicated
+To repeat, this write memory barrier is present if and only if something
+is actually awakened. To see this, consider the following sequence of
+events, where X and Y are both initially zero:
+
+ CPU 1 CPU 2
+ =============================== ===============================
+ X = 1; STORE event_indicated
+ smp_mb(); wake_up();
+ Y = 1; wait_event(wq, Y == 1);
+ wake_up(); load from Y sees 1, no memory barrier
+ load from X might see 0
+
+In contrast, if a wakeup does occur, CPU 2's load from X would be guaranteed
+to see 1.
+
The available waker functions include:
complete();
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