Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

On Fri, Feb 07, 2014 at 04:18:47PM +0100, Peter Zijlstra wrote:

On Fri, Feb 07, 2014 at 01:28:37PM +0100, Peter Zijlstra wrote:

quoted

Anyway, you can do a version with lwarx/stwcx if you're looking get rid
of lharx.

the below seems to compile into relatively ok asm. It can be done better
if you write the entire thing by hand though.

[...]

static inline unsigned int xadd(unsigned int *v, unsigned int i)
{
	int t, ret;
	
	__asm__ __volatile__ (
"1:	lwarx	%0, 0, %4\n"
"	mr	%1, %0\n"
"	add	%0, %3, %0\n"
"	stwcx.	%0, %0, %4\n"
"	bne-	1b\n"
	: "=&r" (t), "=&r" (ret), "+m" (*v)
	: "r" (i), "r" (v)
	: "cc");

	return ret;
}

I don't like this xadd thing -- it's so x86 ;)
x86 has its LOCK prefix, ppc has ll/sc.
That should be reflected somehow IMHO.

Maybe if xadd became mandatory for some kernel library.

void ticket_unlock(tickets_t *lock)
{
	ticket_t tail = lock->tail + 1;

	/*
	 * The store is save against the xadd for it will make the ll/sc fail
	 * and try again. Aside from that PowerISA guarantees single-copy
	 * atomicy for half-word writes.
	 *
	 * And since only the lock owner will ever write the tail, we're good.
	 */
	smp_store_release(&lock->tail, tail);
}

Yeah, let's try that on top of v2 (just posted).
First, I want to see v2 work as nicely as v1 --
compiling a debug kernel takes a while...

	Torsten