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Commit 7817b799 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

documentation: Fix control dependency and identical stores 



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 <nasa4836@gmail.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 92e963f5

Documentation/memory-barriers.txt

+7 −3
Original line number Diff line number Diff line
@@ -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