On Thu, Nov 7, 2013 at 6:31 AM, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > On Thu, Nov 07, 2013 at 04:50:23AM -0800, Michel Lespinasse wrote: >> On Thu, Nov 7, 2013 at 4:06 AM, Linus Torvalds >> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >> > >> > On Nov 7, 2013 6:55 PM, "Michel Lespinasse" <walken@xxxxxxxxxx> wrote: >> >> >> >> Rather than writing arch-specific locking code, would you agree to >> >> introduce acquire and release memory operations ? >> > >> > Yes, that's probably the right thing to do. What ops do we need? Store with >> > release, cmpxchg and load with acquire? Anything else? >> >> Depends on what lock types we want to implement on top; for MCS we would need: >> - xchg acquire (common case) and load acquire (for spinning on our >> locker's wait word) >> - cmpxchg release (when there is no next locker) and store release >> (when writing to the next locker's wait word) >> >> One downside of the proposal is that using a load acquire for spinning >> puts the memory barrier within the spin loop. So this model is very >> intuitive and does not add unnecessary barriers on x86, but it my >> place the barriers in a suboptimal place for architectures that need >> them. > > OK, I will bite... Why is a barrier in the spinloop suboptimal? It's probably not a big deal - all I meant to say is that if you were manually placing barriers, you would probably put one after the loop instead. I don't deal much with architectures where such barriers are needed, so I don't know for sure if the difference means much. > Can't say that I have tried measuring it, but the barrier should not > normally result in interconnect traffic. Given that the barrier is > required anyway, it should not affect lock-acquisition latency. Agree > So what am I missing here? I think you read my second email as me trying to shoot down a proposal - I wasn't, as I really like the acquire/release model and find it easy to program with, which is why I'm proposing it in the first place. I just wanted to be upfront about all potential downsides, so we can consider them and see if they are significant - I don't think they are, but I'm not the best person to judge that as I mostly just deal with x86 stuff. -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>