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

[Guo Ren]

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 :P

-- 
Best Regards
 Guo Ren

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