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documentation: Additional restriction for control dependencies

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Short-circuit booleans are not defences against compilers breaking
your intended control dependencies.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
This commit is contained in:
Paul E. McKenney 2014-10-12 07:55:47 -07:00
parent 74860feed5
commit 8b19d1dead
1 changed files with 18 additions and 0 deletions
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18
Documentation/memory-barriers.txt
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@ -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: