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

[Anup Patel]

> -----Original Message-----
> From: Guo Ren <guoren at kernel.org>
> Sent: 30 March 2021 08:44
> To: Peter Zijlstra <peterz at infradead.org>
> Cc: linux-riscv <linux-riscv at lists.infradead.org>; Linux Kernel Mailing List
> <linux-kernel at vger.kernel.org>; linux-csky at vger.kernel.org; linux-arch
> <linux-arch at vger.kernel.org>; Guo Ren <guoren at linux.alibaba.com>; Will
> Deacon <will at kernel.org>; Ingo Molnar <mingo at redhat.com>; Waiman
> Long <longman at redhat.com>; Arnd Bergmann <arnd at arndb.de>; Anup
> Patel <anup at brainfault.org>
> 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 at infradead.org>
> 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 at hp.com/
> 
> 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.

Regards,
Anup