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author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2014-10-12 07:55:47 -0700 |
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committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2014-11-13 10:34:53 -0800 |
commit | 8b19d1dead8413442ba0ff0b4e19b08f69d2f1b7 (patch) | |
tree | e2dcecc0d8057974aac7b5929f4adfa374351688 /Documentation | |
parent | 74860feed5ed570659e0f3852dd945be5b046038 (diff) | |
download | linux-8b19d1dead8413442ba0ff0b4e19b08f69d2f1b7.tar.gz linux-8b19d1dead8413442ba0ff0b4e19b08f69d2f1b7.tar.bz2 linux-8b19d1dead8413442ba0ff0b4e19b08f69d2f1b7.zip |
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 <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
Diffstat (limited to 'Documentation')
-rw-r--r-- | Documentation/memory-barriers.txt | 18 |
1 files changed, 18 insertions, 0 deletions
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: |