The summary of the "CONTROL DEPENDENCIES" section incorrectly states that barrier() may be used to prevent compiler reordering when more than one leg of the control-dependent "if" statement start with identical stores. This is incorrect at high optimization levels. This commit therefore updates the summary to match the detailed description.
Reported by: Jianyu Zhan <nasa4...@gmail.com> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> --- Documentation/memory-barriers.txt | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 904ee42d078e..e26058d3e253 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -800,9 +800,13 @@ In summary: use smp_rmb(), smp_wmb(), or, in the case of prior stores and later loads, smp_mb(). - (*) If both legs of the "if" statement begin with identical stores - to the same variable, a barrier() statement is required at the - beginning of each leg of the "if" statement. + (*) If both legs of the "if" statement begin with identical stores to + the same variable, then those stores must be ordered, either by + preceding both of them with smp_mb() or by using smp_store_release() + to carry out the stores. Please note that it is -not- sufficient + to use barrier() at beginning of each leg of the "if" statement, + as optimizing compilers do not necessarily respect barrier() + in this case. (*) Control dependencies require at least one run-time conditional between the prior load and the subsequent store, and this -- 2.5.2