memory-barriers: Retain barrier() in fold-to-zero example
The transformation in the fold-to-zero example incorrectly omits the barrier() directive. This commit therefore adds it back in. Reported-by: Pranith Kumar <pranith@gatech.edu> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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@ -679,12 +679,15 @@ equal to zero, in which case the compiler is within its rights to
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transform the above code into the following:
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transform the above code into the following:
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q = ACCESS_ONCE(a);
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q = ACCESS_ONCE(a);
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barrier();
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ACCESS_ONCE(b) = p;
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ACCESS_ONCE(b) = p;
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do_something_else();
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do_something_else();
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This transformation loses the ordering between the load from variable 'a'
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This transformation fails to require that the CPU respect the ordering
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and the store to variable 'b'. If you are relying on this ordering, you
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between the load from variable 'a' and the store to variable 'b'.
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should do something like the following:
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Yes, the barrier() is still there, but it affects only the compiler,
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not the CPU. Therefore, if you are relying on this ordering, you should
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do something like the following:
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q = ACCESS_ONCE(a);
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q = ACCESS_ONCE(a);
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BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
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BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */
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