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SubjectRe: [BUG] segfault in perf-top -- thread refcnt
Em Mon, Mar 30, 2015 at 03:20:59PM +0200, Jiri Olsa escreveu:
> 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 ;-)

yeah, dumb me, hey, its still early here in .br ;-)

- Arnaldo


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