Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation

[Peter Zijlstra]

On Tue, Apr 13, 2021 at 11:22:40AM +0200, Christoph Müllner wrote:

> > For ticket locks you really only needs atomic_fetch_add() and
> > smp_store_release() and an architectural guarantees that the
> > atomic_fetch_add() has fwd progress under contention and that a sub-word
> > store (through smp_store_release()) will fail the SC.
> >
> > Then you can do something like:
> >
> > void lock(atomic_t *lock)
> > {
> >         u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
> >         u16 ticket = val >> 16;
> >
> >         for (;;) {
> >                 if (ticket == (u16)val)
> >                         break;
> >                 cpu_relax();
> >                 val = atomic_read_acquire(lock);
> >         }
> > }
> >
> > void unlock(atomic_t *lock)
> > {
> >         u16 *ptr = (u16 *)lock + (!!__BIG_ENDIAN__);
> >         u32 val = atomic_read(lock);
> >
> >         smp_store_release(ptr, (u16)val + 1);
> > }
> >
> > That's _almost_ as simple as a test-and-set :-) It isn't quite optimal
> > on x86 for not being allowed to use a memop on unlock, since its being
> > forced into a load-store because of all the volatile, but whatever.
> 
> What about trylock()?
> I.e. one could implement trylock() without a loop, by letting
> trylock() fail if the SC fails.
> That looks safe on first view, but nobody does this right now.

Generic code has to use cmpxchg(), and then you get something like this:

bool trylock(atomic_t *lock)
{
	u32 old = atomic_read(lock);

	if ((old >> 16) != (old & 0xffff))
		return false;

	return atomic_try_cmpxchg(lock, &old, old + (1<<16)); /* SC, for RCsc */
}

That will try and do the full LL/SC loop, because it wants to complete
the cmpxchg, but in generic code we have no other option.

(Is this what C11's weak cmpxchg is for?)