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rcu: Eliminate read-modify-write ACCESS_ONCE() calls
RCU contains code of the following forms: ACCESS_ONCE(x)++; ACCESS_ONCE(x) += y; ACCESS_ONCE(x) -= y; Now these constructs do operate correctly, but they really result in a pair of volatile accesses, one to do the load and another to do the store. This can be confusing, as the casual reader might well assume that (for example) gcc might generate a memory-to-memory add instruction for each of these three cases. In fact, gcc will do no such thing. Also, there is a good chance that the kernel will move to separate load and store variants of ACCESS_ONCE(), and constructs like the above could easily confuse both people and scripts attempting to make that sort of change. Finally, most of RCU's read-modify-write uses of ACCESS_ONCE() really only need the store to be volatile, so that the read-modify-write form might be misleading. This commit therefore changes the above forms in RCU so that each instance of ACCESS_ONCE() either does a load or a store, but not both. In a few cases, ACCESS_ONCE() was not critical, for example, for maintaining statisitics. In these cases, ACCESS_ONCE() has been dispensed with entirely. Suggested-by:Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: