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

[Peter Zijlstra]

On Tue, Mar 30, 2021 at 11:13:55AM +0800, Guo Ren wrote:
> 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;
> +}

That entirely depends on the architecture (and cmpxchg() impementation).

There are a number of cases:

 * architecture has cmpxchg() instruction (x86, s390, sparc, etc.).

  - architecture provides fwd progress (x86)
  - architecture requires backoff for progress (sparc)

 * architecture does not have cmpxchg, and implements it using LL/SC.

  and here things get *really* interesting, because while an
  architecture can have LL/SC fwd progress, that does not translate into
  cmpxchg() also having the same guarantees and all bets are off.

The real bummer is that C can do cmpxchg(), but there is no way it can
do LL/SC. And even if we'd teach C how to do LL/SC, it couldn't be
generic because architectures lacking it can't emulate it using
cmpxchg() (there's a fun class of bugs there).

So while the above code might be the best we can do in generic code,
it's really up to the architecture to make it work.