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

[Peter Zijlstra]

On Mon, Apr 12, 2021 at 11:54:55PM +0200, Christoph Müllner wrote:
> On Mon, Apr 12, 2021 at 7:33 PM Palmer Dabbelt <palmer at dabbelt.com> wrote:

> > My plan is to add a generic ticket-based lock, which can be selected at
> > compile time.  It'll have no architecture dependencies (though it'll
> > likely have some hooks for architectures that can make this go faster).
> > Users can then just pick which spinlock flavor they want, with the idea
> > being that smaller systems will perform better with ticket locks and
> > larger systems will perform better with queued locks.  The main goal
> > here is to give the less widely used architectures an easy way to have
> > fair locks, as right now we've got a lot of code duplication because any
> > architecture that wants ticket locks has to do it themselves.
> 
> In the case of LL/SC sequences, we have a maximum of 16 instructions
> on RISC-V. My concern with a pure-C implementation would be that
> we cannot guarantee this (e.g. somebody wants to compile with -O0)
> and I don't know of a way to abort the build in case this limit exceeds.
> Therefore I have preferred inline assembly for OpenSBI (my initial idea
> was to use closure-like LL/SC macros, where you can write the loop
> in form of C code).

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.