rcu: Add comment on evaluate-once properties of rcu_assign_pointer(). (ac7c8e3d) · Commits · 戴 / test

The rcu_assign_pointer() macro, as with most cpp macros, must not evaluate its argument more than once. And it in fact does not. But this might not be obvious to the casual observer, because one of the arguments appears no less than three times. However, but one expansion is only visible to sparse (__CHECKER__), and one lives inside a typeof (where it will never be evaluated), so this is in fact safe. This commit therefore adds a comment making this explicit. Reported-by:

Josh Triplett <josh@joshtriplett.org> Signed-off-by:

Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by:

Josh Triplett <josh@joshtriplett.org>

include/linux/rcupdate.h

+8 −0

Original line number	Diff line number	Diff line
		@@ -911,6 +911,14 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
		* rcu_assign_pointer() is a very bad thing that results in
		* impossible-to-diagnose memory corruption. So please be careful.
		* See the RCU_INIT_POINTER() comment header for details.
		*
		* Note that rcu_assign_pointer() evaluates each of its arguments only
		* once, appearances notwithstanding. One of the "extra" evaluations
		* is in typeof() and the other visible only to sparse (__CHECKER__),
		* neither of which actually execute the argument. As with most cpp
		* macros, this execute-arguments-only-once property is important, so
		* please be careful when making changes to rcu_assign_pointer() and the
		* other macros that it invokes.
		*/
		#define rcu_assign_pointer(p, v) \
		__rcu_assign_pointer((p), (v), __rcu)

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