On Tue, 24 Jul 2007, Nick Piggin wrote: > > > For the purpose of this discussion (Linux memory > > > barrier semantics, on WB memory), it is true of CPU > > > and compiler barriers. > > > > On later Intel processors, if the memory address range being referenced > > (and say written to) by the (locked) instruction is in the cache of a > > CPU, then it will not assert the LOCK signal on the system bus -- > > thus not assume exclusive use of shared memory. So other CPUs are free > > to modify (other) memory at that point. Cache coherency will still > > ensure _that_ (locked) memory area is still updated atomically, though. > > The system bus does not need to be serialised because the CPU already > holds the cacheline in exclusive state. That *is* the cache coherency > protocol. > > The memory ordering is enforced by the CPU effectively preventing > speculative loads to pass the locked instruction and ensuring all > stores reach the cache before it is executed. (I say effectively > because the CPU might do clever tricks without you knowing).
Looks like when you said "CPU memory barrier extends to all memory references" you were probably referring to a _given_ CPU ... yes, that statement is correct in that case. > > > Are you saying that it is OK for the store to var to > > > be reordered below the clear_bit? If not, what are you > > > saying? > > > > > > I might be making a radical turn-around here, but all of a > > sudden I think it's actually a good idea to put a complete > > memory clobber in set_bit/clear_bit and friends themselves, > > and leave the "__" variants as they are. > > Why? Well, why not. Callers who don't want/need any guarantees whatsoever can still use __foo() -- for others, it makes sense to just use foo() and get *both* the compiler and CPU barrier semantics -- I think that's the behaviour most callers would want anyway. Satyam - 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/
