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Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 40 |
1 files changed, 29 insertions, 11 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 4af4cea8cff0..7ee2ae6d5451 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -121,22 +121,22 @@ For example, consider the following sequence of events: The set of accesses as seen by the memory system in the middle can be arranged in 24 different combinations: - STORE A=3, STORE B=4, x=LOAD A->3, y=LOAD B->4 - STORE A=3, STORE B=4, y=LOAD B->4, x=LOAD A->3 - STORE A=3, x=LOAD A->3, STORE B=4, y=LOAD B->4 - STORE A=3, x=LOAD A->3, y=LOAD B->2, STORE B=4 - STORE A=3, y=LOAD B->2, STORE B=4, x=LOAD A->3 - STORE A=3, y=LOAD B->2, x=LOAD A->3, STORE B=4 - STORE B=4, STORE A=3, x=LOAD A->3, y=LOAD B->4 + STORE A=3, STORE B=4, y=LOAD A->3, x=LOAD B->4 + STORE A=3, STORE B=4, x=LOAD B->4, y=LOAD A->3 + STORE A=3, y=LOAD A->3, STORE B=4, x=LOAD B->4 + STORE A=3, y=LOAD A->3, x=LOAD B->2, STORE B=4 + STORE A=3, x=LOAD B->2, STORE B=4, y=LOAD A->3 + STORE A=3, x=LOAD B->2, y=LOAD A->3, STORE B=4 + STORE B=4, STORE A=3, y=LOAD A->3, x=LOAD B->4 STORE B=4, ... ... and can thus result in four different combinations of values: - x == 1, y == 2 - x == 1, y == 4 - x == 3, y == 2 - x == 3, y == 4 + x == 2, y == 1 + x == 2, y == 3 + x == 4, y == 1 + x == 4, y == 3 Furthermore, the stores committed by a CPU to the memory system may not be @@ -694,6 +694,24 @@ Please note once again that the stores to 'b' differ. If they were identical, as noted earlier, the compiler could pull this store outside of the 'if' statement. +You must also be careful not to rely too much on boolean short-circuit +evaluation. Consider this example: + + q = ACCESS_ONCE(a); + if (a || 1 > 0) + ACCESS_ONCE(b) = 1; + +Because the second condition is always true, the compiler can transform +this example as following, defeating control dependency: + + q = ACCESS_ONCE(a); + ACCESS_ONCE(b) = 1; + +This example underscores the need to ensure that the compiler cannot +out-guess your code. More generally, although ACCESS_ONCE() does force +the compiler to actually emit code for a given load, it does not force +the compiler to use the results. + Finally, control dependencies do -not- provide transitivity. This is demonstrated by two related examples, with the initial values of x and y both being zero: |