On Fri, 2013-02-22 at 14:54 +0100, Ingo Molnar wrote: > * Peter Zijlstra <[email protected]> wrote: > > > On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote: > > > > atomic64_read() and atomic64_set() are supposed to take care > > > of that, without even the need for _inc() or _add() parts > > > that use LOCK. > > > > Are you sure? Generally atomic*_set() is not actually an > > atomic operation. > > as per Documentation/atomic_ops.h:
I think the interesting part is: "The setting is atomic in that the return values of the atomic operations by all threads are guaranteed to be correct reflecting either the value that has been set with this operation or set with another operation. A proper implicit or explicit memory barrier is needed before the value set with the operation is guaranteed to be readable with atomic_read from another thread." Which does give us the wanted guarantee, however: I checked arch/x86/include/asm/atomic64_32.h and we use cmpxchg8b for everything from _set() to _read(), which translates into 'horridly stupendifyingly slow' for a number of machines, but coherent. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
