lkml.org 
[lkml]   [2006]   [Nov]   [19]   [last100]   RSS Feed
Views: [more markup]  [less markup]  [headers]  [forward] 
 
Messages in this thread
/
DateMon, 20 Nov 2006 00:17:31 +0300
FromOleg Nesterov <>
SubjectRe: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
On 11/19, Alan Stern wrote:
> On Sun, 19 Nov 2006, Oleg Nesterov wrote:
> 
> > > What happens if synchronize_xxx manages to execute inbetween 
> > > xxx_read_lock's
> > > 
> > >  		idx = sp->completed & 0x1;
> > >  		atomic_inc(sp->ctr + idx);
> > > 
> > > statements?
> > 
> > Oops. I forgot about explicit mb() before sp->completed++ in synchronize_xxx().
> > 
> > So synchronize_xxx() should do
> > 
> > 	smp_mb();
> > 	idx = sp->completed++ & 0x1;
> > 
> > 	for (;;) { ... }
> > 
> > >               You see, there's no way around using synchronize_sched().
> > 
> > With this change I think we are safe.
> > 
> > If synchronize_xxx() increments ->completed in between, the caller of
> > xxx_read_lock() will see all memory ops (started before synchronize_xxx())
> > completed. It is ok that synchronize_xxx() returns immediately.
> 
> Yes, the reader will see a consistent picture, but it will have 
> incremented the wrong element of sp->ctr[].  What happens if another 
> synchronize_xxx() occurs while the reader is still running?

It will wait for xxx_read_unlock() on reader's side. And for this reason
this idx in fact is not exactly wrong :)

Oleg.

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-11-19 22:21    [from the cache]
©2003-2008