Re: Serious problem with ticket spinlocks on ia64

[Petr Tesarik <ptesarik@suse.cz>]

On Monday 30 of August 2010 20:17:25 Luck, Tony wrote:
> > I may tinker with this test a bit to include some short random
> > amounts of hold-time for the lock, and delays between attempts
> > to acquire it (to make it look more like a contended kernel lock
> > and less like a continuous queue of processes trading around a
> > lock that is never free
>
> I've been iterating ... adding new bits to try to reproduce the
> kernel environment:

Hi Tony,

I've been also playing with my test case, and I haven't been able to reproduce 
it in user-space either. One thing I noticed was the apparently incorrect use 
of ALAT. The generated code for _spin_lock_irq contains:

invala;;
ld4.c.nc r11=[r32]
// Other instructions not affecting r20
ld4.c.nc r20=[r32]

IIUC, the subsequent compare can use an undefined value (r20 is not modified 
anywhere in this function, except by the ld4.c.nc, but that happens only on 
an ALAT miss, right?).

I changed the corresponding code in __ticket_spin_lock to:

   asm volatile ("ld4.c.nc %0=[%1]" : "+r"(serve) : "r"(p) : "memory");

(NB the "+r" constraint instead of "=r")

The generated code now re-uses r15. Unfortunately, Hedi's test case still 
fails for me. :(

Petr Tesarik