linux-kernel - Re: [PATCH tip 1/1] perf_counter tools: Add locking to perf top

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Date:	Fri, 29 May 2009 17:33:22 -0300
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH tip 1/1] perf_counter tools: Add locking to perf top

Em Fri, May 29, 2009 at 10:22:17PM +0200, Peter Zijlstra escreveu:
> On Fri, 2009-05-29 at 17:03 -0300, Arnaldo Carvalho de Melo wrote:
> >         /* Sort the active symbols */
> > -       list_for_each_entry_safe(syme, n, &active_symbols, node) {
> > -               if (syme->count[0] != 0) {
> > +       pthread_mutex_lock(&active_symbols_lock);
> > +       syme = list_entry(active_symbols.next, struct sym_entry, node);
> > +       pthread_mutex_unlock(&active_symbols_lock);
> > +
> > +       list_for_each_entry_safe_from(syme, n, &active_symbols, node) {
> > +               syme->snap_count = syme->count[0];
> > +               if (syme->snap_count != 0) {
> > +                       syme->weight = sym_weight(syme);
> 
> That looks wrong, you basically do a fancy cast while holding the lock,
> then you overwrite the variable doing a list iteration without holding
> the lock.
> 
> If list_add and list_del are under a lock, the iteration should be too.

Look closer :)

1) List insertion is only done at the head and by the other thread, thus
the lock above. The other thread will only mess with the above
syme->node.prev when inserting a new head, never with .next.

2) List deletion is only done after taking the lock, and on the above
thread.

Only problem probably is to access syme->count[0], that on some
architectures may not be atomic.

- Arnaldo
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