On Mon, Mar 29, 2021 at 3:50 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Sat, Mar 27, 2021 at 06:06:38PM +0000, guoren@xxxxxxxxxx wrote: > > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx> > > > > Some architectures don't have sub-word swap atomic instruction, > > they only have the full word's one. > > > > The sub-word swap only improve the performance when: > > NR_CPUS < 16K > > * 0- 7: locked byte > > * 8: pending > > * 9-15: not used > > * 16-17: tail index > > * 18-31: tail cpu (+1) > > > > The 9-15 bits are wasted to use xchg16 in xchg_tail. > > > > Please let architecture select xchg16/xchg32 to implement > > xchg_tail. > > So I really don't like this, this pushes complexity into the generic > code for something that's really not needed. > > Lots of RISC already implement sub-word atomics using word ll/sc. > Obviously they're not sharing code like they should be :/ See for > example arch/mips/kernel/cmpxchg.c. I see, we've done two versions of this: - Using cmpxchg codes from MIPS by Michael - Re-write with assembly codes by Guo But using the full-word atomic xchg instructions implement xchg16 has the semantic risk for atomic operations. I don't think export xchg16 in a none-sub-word atomic machine is correct. > > Also, I really do think doing ticket locks first is a far more sensible > step. NACK by Anup -- Best Regards Guo Ren ML: https://lore.kernel.org/linux-csky/
