From 8b19d1dead8413442ba0ff0b4e19b08f69d2f1b7 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sun, 12 Oct 2014 07:55:47 -0700 Subject: documentation: Additional restriction for control dependencies Short-circuit booleans are not defences against compilers breaking your intended control dependencies. Signed-off-by: Paul E. McKenney Reviewed-by: Pranith Kumar --- Documentation/memory-barriers.txt | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) (limited to 'Documentation/memory-barriers.txt') diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 22a969cdd476..1073e019ef06 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -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: -- cgit v1.2.3 From 8ab8b3e1837fc580b30263ed3c44dc34798714d9 Mon Sep 17 00:00:00 2001 From: Pranith Kumar Date: Tue, 2 Sep 2014 23:34:29 -0400 Subject: documentation: memory-barriers.txt: Correct example for reorderings Correct the example of memory orderings in memory-barriers.txt Commit 615cc2c9cf95 "Documentation/memory-barriers.txt: fix important typo re memory barriers" changed the assignment to x and y. Change the rest of the example to match this change. Reported-by: Ganesh Rapolu Signed-off-by: Pranith Kumar Signed-off-by: Paul E. McKenney --- Documentation/memory-barriers.txt | 22 +++++++++++----------- 1 file changed, 11 insertions(+), 11 deletions(-) (limited to 'Documentation/memory-barriers.txt') diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 1073e019ef06..f7fa63508aba 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 -- cgit v1.2.3