Messages in this thread | | | From | Guo Ren <> | Date | Mon, 29 Mar 2021 20:01:41 +0800 | Subject | Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32 |
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On Mon, Mar 29, 2021 at 7:26 PM Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Mar 29, 2021 at 07:19:29PM +0800, Guo Ren wrote: > > On Mon, Mar 29, 2021 at 3:50 PM Peter Zijlstra <peterz@infradead.org> wrote: > > > > > > On Sat, Mar 27, 2021 at 06:06:38PM +0000, guoren@kernel.org wrote: > > > > From: Guo Ren <guoren@linux.alibaba.com> > > > > > > > > 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. > > What? -ENOPARSE
u32 a = 0x55aa66bb; u16 *ptr = &a;
CPU0 CPU1 ========= ========= xchg16(ptr, new) while(1) WRITE_ONCE(*(ptr + 1), x);
When we use lr.w/sc.w implement xchg16, it'll cause CPU0 deadlock.
> > > > Also, I really do think doing ticket locks first is a far more sensible > > > step. > > NACK by Anup > > Who's he when he's not sending NAKs ? We've talked before: https://lore.kernel.org/linux-riscv/CAAhSdy1JHLUFwu7RuCaQ+RUWRBks2KsDva7EpRt8--4ZfofSUQ@mail.gmail.com/T/#t
-- Best Regards Guo Ren
ML: https://lore.kernel.org/linux-csky/
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