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

[Namhyung Kim <namhyung@kernel.org>]

On Mon, Mar 30, 2015 at 9:56 PM, Jiri Olsa <jolsa@redhat.com> wrote:
> 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:
>> > > >
>> > > > SNIP
>> > > >
>> > > > > >
>> > > > > > 2 things:
>> > > > > > 1. let run for a long time. go about using the server. do lots of builds,
>> > > > > > etc. it takes time
>> > > > > >
>> > > > > > 2. use a box with a LOT of cpus (1024 in my case)
>> > > > > >
>> > > > > > Make sure ulimit is set to get the core.
>> > > > >
>> > > > > reproduced under 24 cpu box with kernel build (make -j25)
>> > > > > running on background.. will try to look closer
>> > > > >
>> > > > > perf: Segmentation fault
>> > > > > -------- backtrace --------
>> > > > > ./perf[0x4fd79b]
>> > > > > /lib64/libc.so.6(+0x358f0)[0x7f9cbff528f0]
>> > > > > ./perf(thread__put+0x5b)[0x4b1a7b]
>> > > > > ./perf(hists__delete_entries+0x70)[0x4c8670]
>> > > > > ./perf[0x436a88]
>> > > > > ./perf[0x4fa73d]
>> > > > > ./perf(perf_evlist__tui_browse_hists+0x97)[0x4fc437]
>> > > > > ./perf[0x4381d0]
>> > > > > /lib64/libpthread.so.0(+0x7ee5)[0x7f9cc1ff2ee5]
>> > > > > /lib64/libc.so.6(clone+0x6d)[0x7f9cc0011b8d]
>> > > > > [0x0]
>> > > >
>> > > > looks like race among __machine__findnew_thread and thread__put
>> > > > over the machine->threads rb_tree insert/removal
>> > > >
>> > > > is there a reason why thread__put does not erase itself from machine->threads?
>> >
>> > that was the reason.. we do this separately.. not in thread__put..
>> > is there a reason for this? ;-)
>> >
>> > testing attached patch..
>> >
>> > jirka
>> >
>> >
>> > ---
>> > diff --git a/tools/perf/util/build-id.c b/tools/perf/util/build-id.c
>> > index f7fb258..966564a 100644
>> > --- a/tools/perf/util/build-id.c
>> > +++ b/tools/perf/util/build-id.c
>> > @@ -60,7 +60,6 @@ static int perf_event__exit_del_thread(struct perf_tool *tool __maybe_unused,
>> >                 event->fork.ppid, event->fork.ptid);
>> >
>> >     if (thread) {
>> > -           rb_erase(&thread->rb_node, &machine->threads);
>> >             if (machine->last_match == thread)
>> >                     thread__zput(machine->last_match);
>> >             thread__put(thread);
>> > diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
>> > index e335330..a8443ef 100644
>> > --- a/tools/perf/util/machine.c
>> > +++ b/tools/perf/util/machine.c
>> > @@ -30,6 +30,7 @@ int machine__init(struct machine *machine, const char *root_dir, pid_t pid)
>> >     dsos__init(&machine->kernel_dsos);
>> >
>> >     machine->threads = RB_ROOT;
>> > +   pthread_mutex_init(&machine->threads_lock, NULL);
>> >     INIT_LIST_HEAD(&machine->dead_threads);
>> >     machine->last_match = NULL;
>> >
>> > @@ -380,10 +381,13 @@ static struct thread *__machine__findnew_thread(struct machine *machine,
>> >     if (!create)
>> >             return NULL;
>> >
>> > -   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 ;-)
>
>>
>> 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

Agreed.

AFAIK the reason we do ref-counting is to cleanup dead/exited thread
for live session like perf top.  In that case we can somehow mark
to-be-deleted thread and kill it in a safe time/place..

Thanks,
Namhyung