On Wed, Dec 06, 2017 at 12:09:36AM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 10:57:00PM +0100, Peter Zijlstra wrote: > > On Tue, Dec 05, 2017 at 11:24:49PM +0200, Michael S. Tsirkin wrote: > > > READ_ONCE is really all over the place (some code literally replaced all > > > memory accesses with READ/WRITE ONCE). > > > > Yeah, so? > > Oh my point was I can't just look for READ_ONCE and go > *that's the pair*. there are too many of these. > At Paul's suggestion I will document the pairing *this read once has a > barrier that is paired with that barrier*. > > > Complain to the compiler people for forcing us into that. > > In some cases when you end up with all accesses > going through read/write once volatile just might better. That is in fact what the jiffies counter does. But you lose READ_ONCE()'s automatic handling of DEC Alpha when you take that approach. > > > Would an API like WRITE_POINTER()/smp_store_pointer make sense, > > > and READ_POINTER for symmetry? > > > > No, the whole point of the exercise was to get away from the fact that > > dependent loads are special. > > It's a pity that dependent stores are still special. We can make READ_ONCE() not be special at zero cost on non-Alpha systems, but both smp_wmb() and smp_store_release() are decidedly not free of added overhead. Thanx, Paul
