On Tue, Dec 17, 2013 at 11:27:29AM -0800, Linus Torvalds wrote: > On Tue, Dec 17, 2013 at 11:21 AM, Paul E. McKenney > <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > Looks like xadd() is x86-specific, but this is common code. One > > approach would be to do xadd() for other arches, another approach > > would be to make .rw be an atomic_t rather than a u32. Making it > > be atomic_t is probably easiest. (The cmpxchg()s would then need > > to be atomic_cmpxchg().) > > Note that "xadd()" has different semantics from "atomic_add_return()". Gah, that one always trips me up. :-/ > xadd() returns the original value, while atomic_add_return() returns > the result of the addition. > > In this case, we seem to want the xadd() semantics. I guess we can use > "atomic_add_return(val,&atomic)-val" and just assume that the compiler > gets it right (with the addition and the subtraction cancelling out). That seems like it would work well. > Or maybe we should have a "atomic_add_return_original()" with xadd > semantics? My lazy side prefers the autocancellation. ;-) But yes, there are a number of architectures (including ARM and Power) where the compiler would have to be very tricky to reach into an asm to do the cancellation. So perhaps a generic atomic_add_return_original() that is defined in terms of atomic_add_return() as you say above, allowing architectures to override with more-efficient implementations? The same could be done for add_smp() and xadd(), for that matter. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
