Re: [PATCH] Fix a race in pid generation that causes pids to bereused immediately.

[Andrew Morton]

On Tue, 15 Jun 2010 13:26:08 +1000 Paul Mackerras <paulus@xxxxxxxxx> wrote:

> On Mon, Jun 14, 2010 at 06:55:56PM -0700, Andrew Morton wrote:
> 
> > > kernel/sched_clock.c:   if (cmpxchg64(&scd->clock, old_clock, clock) != old_cloc
> > 
> > I guess that'll flush out any stragglers.
> 
> And break most non-x86 32-bit architectures, including 32-bit powerpc.

If CONFIG_SMP=y, yes.  On UP there's a generic implementation
(include/asm-generic/cmpxchg-local.h, include/asm-generic/cmpxchg.h)

> Fortunately that code is only used if CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
> is set, and it looks like only x86 and ia64 set it.
> 

If that happens then the best fix is for those architectures to get
themselves a cmpxchg64().  Unless for some reason it's simply
unimplementable?  Worst case I guess one could use a global spinlock. 
Second-worst-case: hashed spinlocks.
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