Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

[Alan Stern <stern@rowland.harvard.edu>]

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?

Alan

-
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/