Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

[Guo Ren]

On Mon, Mar 29, 2021 at 7:26 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@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.
>
> 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@xxxxxxxxxxxxxx/T/#t

-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/