linux-kernel - Re: [BUG] segfault in perf-top -- thread refcnt

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Date:	Mon, 30 Mar 2015 15:20:59 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] segfault in perf-top -- thread refcnt

On Mon, Mar 30, 2015 at 10:07:08AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Mar 30, 2015 at 02:56:31PM +0200, Jiri Olsa escreveu:
> > On Mon, Mar 30, 2015 at 09:48:52PM +0900, Namhyung Kim wrote:
> > > Hi Jiri,
> > > 
> > > On Mon, Mar 30, 2015 at 01:49:07PM +0200, Jiri Olsa wrote:
> > > > On Mon, Mar 30, 2015 at 01:21:08PM +0200, Jiri Olsa wrote:
> > > > > On Mon, Mar 30, 2015 at 12:22:20PM +0200, Jiri Olsa wrote:
> > > > > > On Mon, Mar 30, 2015 at 10:07:37AM +0200, Jiri Olsa wrote:
> > > > -	th = thread__new(pid, tid);
> > > > +	th = thread__new(machine, pid, tid);
> > > >  	if (th != NULL) {
> > > > +
> > > > +		pthread_mutex_lock(&machine->threads_lock);
> > > >  		rb_link_node(&th->rb_node, parent, p);
> > > >  		rb_insert_color(&th->rb_node, &machine->threads);
> > > > +		pthread_mutex_unlock(&machine->threads_lock);
> > > 
> > > I think you also need to protect the rb tree traversal above.
> > 
> > yep, I already have another version.. but it blows on another place ;-)
> 
> Well, why? The point of refcounting is that the structure will not go
> away while we have it in the rbtree.
> 
> Or are you talking about two threads trying to insert entries in the
> rbtree? Can you point where this can happen?

yep, as I wrote in previous email:

> looks like race among __machine__findnew_thread and thread__put
> over the machine->threads rb_tree insert/removal

update thread:
  perf_event__process_sample
    perf_event__preprocess_sample
      machine__findnew_thread
        traverse machine->threads

display thread:
   perf_top__sort_new_samples
      hists__delete_entries
        ...
        hist_entry__delete(struct hist_entry *he)
          thread__zput(he->thread);

>  
> > > But this makes every sample processing grabs and releases the lock so
> > > might cause high overhead.  It can be a problem if such processing is
> > > done parallelly like my multi-thread work. :-/
>  
> > yep.. perhaps instead of more locking we need to find a way where
> > only single thread do the update on hists/threads
> 
> But that should be really rare, no? The problem is to search in one
> thread (the fast path) and inserting entries (slow path), no?

there're many new threads on kernel make -j25 workload ;-)

jirka
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