On Wed, Nov 12, 2014 at 07:23:22AM -0800, Alexander Duyck wrote: > > On 11/12/2014 02:15 AM, Peter Zijlstra wrote: > >On Tue, Nov 11, 2014 at 01:12:32PM -0800, Alexander Duyck wrote: > >>>Minor nit on naming, but load_acquire would match what we do with barriers, > >>>where you simply drop the smp_ prefix if you want the thing to work on UP > >>>systems too. > >>The problem is this is slightly different, load_acquire in my mind would use > >>a mb() call, I only use a rmb(). That is why I chose read_acquire as the > >>name. > >acquire is not about rmb vs mb, do read up on > >Documentation/memory-barriers.txt. Its a distinctly different semantic. > >Some archs simply lack the means of implementing this semantics and have > >to revert to mb (stronger is always allowed). > > > >Using the read vs load to wreck the acquire semantics is just insane. > > Actually I have been reading up on it as I wasn't familiar with C11. C11 is _different_ although somewhat related. > Most > of what I was doing was actually based on the documentation in barriers.txt > which was referring to memory operations not loads/stores when referring to > the acquire/release so I assumed the full memory barrier was required. I > wasn't aware that smp_load_acquire was only supposed to be ordering loads, > or that smp_ store_release only applied to stores. It does not.. an ACQUIRE is a semi-permeable barrier that doesn't allow LOADs nor STOREs that are issued _after_ it to appear to happen _before_. The RELEASE is the opposite number, it ensures LOADs and STOREs that are issued _before_ cannot happen _after_. This typically matches locking, where a lock (mutex_lock, spin_lock etc..) have ACQUIRE semantics and the unlock RELEASE. Such that: spin_lock(); a = 1; b = x; spin_unlock(); guarantees all LOADs (x) and STORESs (a,b) happen _inside_ the lock region. What they do not guarantee is: y = 1; spin_lock() a = 1; b = x; spin_unlock() z = 4; An order between y and z, both are allowed _into_ the region and can cross there like: spin_lock(); ... z = 4; y = 1; ... spin_unlock(); The only 'open' issue at the moment is if RELEASE+ACQUIRE := MB. Currently we say this is not so, but Will (and me) would very much like this to be so -- PPC64 being the only arch that actually makes this distinction. -- 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
