On Thu, Jan 14, 2016 at 12:04:45PM +0000, Will Deacon wrote: > On Wed, Jan 13, 2016 at 12:58:22PM -0800, Leonid Yegoshin wrote: > > On 01/13/2016 12:48 PM, Peter Zijlstra wrote: > > >On Wed, Jan 13, 2016 at 11:02:35AM -0800, Leonid Yegoshin wrote: > > > > > >>I ask HW team about it but I have a question - has it any relationship with > > >>replacing MIPS SYNC with lightweight SYNCs (SYNC_WMB etc)? > > >Of course. If you cannot explain the semantics of the primitives you > > >introduce, how can we judge the patch. > > > > > > > > You missed a point - it is a question about replacement of SYNC with > > lightweight primitives. It is NOT a question about multithread system > > behavior without any SYNC. The answer on a latest Will's question lies in > > different area. > > The reason we (Peter and I) care about this isn't because we enjoy being > obstructive. It's because there is a whole load of core (i.e. portable) > kernel code that is written to the *kernel* memory model. For example, > the scheduler, RCU, mutex implementations, perf, drivers, you name it. > > Consequently, it's important that the architecture back-ends implement > these portable primitives (e.g. smp_mb()) in a way that satisfies the > kernel memory model so that core code doesn't need to worry about the > underlying architecture for synchronisation purposes. You could turn > around and say "but if MIPS gets it wrong, then that's MIPS's problem", > but actually not having a general understanding of the ordering guarantees > provided by each architecture makes it very difficult for us to extend > the kernel memory model in such a way that it can be implemented > efficiently across the board *and* relied upon by core code. What Will said! Yes, you can cut corners within MIPS architecture-specific code, but primitives that are used in the core kernel really do need to work as expected. Thanx, Paul > The virtio patch at the start of the thread doesn't particularly concern > me. It's the other patches you linked to that implement acquire/release > that have me worried. > > Will >
