The dereference of a volatile-qualified pointer does not guarantee that it cannot be optimized by the compiler to be loaded multiple times into memory even if assigned to a local variable by C99 or any previous C standard. Clarify the comment of ACCESS_ONCE() to state explicitly that its current form relies on the compiler's implementation to work correctly. Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx> --- include/linux/compiler.h | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/include/linux/compiler.h b/include/linux/compiler.h --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -348,6 +348,15 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); * merging, or refetching absolutely anything at any time. Its main intended * use is to mediate communication between process-level code and irq/NMI * handlers, all running on the same CPU. + * + * The current implementation of ACCESS_ONCE() works in all mainstream C + * compilers (including of course gcc), but is not guaranteed by the C standard. + * However, there is a lot of software that depends on the semantics of volatile + * casts, so we have good reason to believe that there will always be a way of + * implementing ACCESS_ONCE(). The implementation of ACCESS_ONCE() might well + * change to track the C standard and the mainstream C compilers, so if you copy + * this definition into some other code base, be sure to check back here + * periodically for changes. */ #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>