[lttng-dev] UST Hanging at Tracepoint

Tue Jun 12 20:21:28 EDT 2012

* Mathieu Desnoyers (mathieu.desnoyers at efficios.com) wrote:
> * Burton, Michael (mburton at ciena.com) wrote:
> > Mathieu,
> > 
> > I think there is a deadlock scenario in UST, which has been causing my problem.
> 
> Good catch !
> 
> > 
> > sessiond is started as root:
> >     - creates global sockets ONLY
> >     - DOES NOT CREATE shm in $HOME/lttng-ust-wait-<uid>
> > 
> > application linked against ust is run as root:
> >   - in lttng_ust_init constructor
> >   - ust_listener_thread  (local_apps)
> >     - fails to connect to local_apps in $HOME/.lttng (as expected)
> >     - prev_connect_failed=1
> >     - ust_unlock()
> >     - restart
> >     - wait_for_sessiond()
> >  -->  - ust_lock()
> >  |      - get_map_shm()
> >  |        - get_wait_shm()
> > DEADLOCK    - shm_open() FAILS (not created by sessiond when run by root)
> >  |            - fork() (trying to create shared memory itself)
> >  |              - ust_before_fork()
> >  ------------>    - ust_lock()
> > 
> > 
> > You should be able to create this with an empty main, with no
> > tracepoints.  As long as sessiond is started as root so
> > $HOME/lttng-ust-wait-<uid> is not created.  You can also make the
> > lttng-ust constructor (lttng_ust_init) wait forever and then you'll be
> > able to see the deadlock in gdb without even leaving the
> > lttng_ust_init constructor.
> 
> Ah, I see. This deadlock is caused by the interaction between
> liblttng-ust-libc-wrapper and liblttng-ust (the fork override is
> performed by liblttng-ust-libc-wrapper). I'll have to look into this
> issue further. More on that soon.

Actually, I meant that it comes from interaction between
liblttng-ust-fork and liblttng-ust (sorry about the confusion above).
I'll see if I can provide a direct access to the fork() symbol to
liblttng-ust, using a weak symbol so it is defined even when
liblttng-ust-fork is not linked.