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

[Guo Ren]

On Tue, Mar 30, 2021 at 12:54 PM Anup Patel <Anup.Patel@xxxxxxx> wrote:
>
>
>
> > -----Original Message-----
> > From: Guo Ren <guoren@xxxxxxxxxx>
> > Sent: 30 March 2021 08:44
> > To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: linux-riscv <linux-riscv@xxxxxxxxxxxxxxxxxxx>; Linux Kernel Mailing List
> > <linux-kernel@xxxxxxxxxxxxxxx>; linux-csky@xxxxxxxxxxxxxxx; linux-arch
> > <linux-arch@xxxxxxxxxxxxxxx>; Guo Ren <guoren@xxxxxxxxxxxxxxxxx>; Will
> > Deacon <will@xxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Waiman
> > Long <longman@xxxxxxxxxx>; Arnd Bergmann <arnd@xxxxxxxx>; Anup
> > Patel <anup@xxxxxxxxxxxxxx>
> > Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add
> > ARCH_USE_QUEUED_SPINLOCKS_XCHG32
> >
> > On Mon, Mar 29, 2021 at 8:50 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > wrote:
> > >
> > > On Mon, Mar 29, 2021 at 08:01:41PM +0800, Guo Ren wrote:
> > > > 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.
> > >
> > > Then I think your LL/SC is broken.
> > >
> > > That also means you really don't want to build super complex locking
> > > primitives on top, because that live-lock will percolate through.
> > Do you mean the below implementation has live-lock risk?
> > +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> > +{
> > +       u32 old, new, val = atomic_read(&lock->val);
> > +
> > +       for (;;) {
> > +               new = (val & _Q_LOCKED_PENDING_MASK) | tail;
> > +               old = atomic_cmpxchg(&lock->val, val, new);
> > +               if (old == val)
> > +                       break;
> > +
> > +               val = old;
> > +       }
> > +       return old;
> > +}
> >
> >
> > >
> > > Step 1 would be to get your architecute fixed such that it can provide
> > > fwd progress guarantees for LL/SC. Otherwise there's absolutely no
> > > point in building complex systems with it.
> >
> > Quote Waiman's comment [1] on xchg16 optimization:
> >
> > "This optimization is needed to make the qspinlock achieve performance
> > parity with ticket spinlock at light load."
> >
> > [1] https://lore.kernel.org/kvm/1429901803-29771-6-git-send-email-
> > Waiman.Long@xxxxxx/
> >
> > So for a non-xhg16 machine:
> >  - ticket-lock for small numbers of CPUs
> >  - qspinlock for large numbers of CPUs
> >
> > Okay, I'll put all of them into the next patch
>
> I would suggest to have separate Kconfig opitons for ticket spinlock
> in Linux RISC-V which will be disabled by default. This means Linux
> RISC-V will use qspinlock by default and use ticket spinlock only when
> ticket spinlock kconfig is enabled.
OK

>
> Regards,
> Anup



-- 
Best Regards
 Guo Ren

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