[lttng-dev] lttng create freezes sometimes

[Yannick Brosseau]

The first thing you should do to provide more information is to run the
lttng-sessiond by itself, with full logging activated.

To do this, as the same user you were running lttng, just run
lttng-sessiond -vvv
and then run your lttng commands as usual. (Make sure you do not have
other lttng-sessiond running)

With that you will see the full output of the session daemon. We might
see some errors there.

On 2012-01-04 12:28, Sébastien Barthélémy wrote:
> Hello all,
>
> I'd like to report that sometimes lttng freezes while creating a session.
>
> This is kind of sporadic, but happens almost half the times on this
> target. I have not seen this behaviour on two other targets. It might
> well be a cross build trick or so.
>
> I straced a session, here is the interesting excerpt (full strace attached)
>
> $ killall lttng-sessiond
> $ strace lttng create
> ...
> write(2, "Spawning a session daemon\n", 26Spawning a session daemon
> ) = 26
> clone(child_stack=0,
> flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
> child_tidptr=0xb7ef3728) = 4044
> --- SIGCHLD (Child exited) @ 0 (0) ---
> sigreturn()                             = ? (mask now [])
> pause(
>
> I'm not sure what happens. It seems to me that
>
> 1) lttng-sessiond sends SIGCHLD,
> 2) lttng catches the signal
> 3) lttng is still locked in pause()
>
> But I may be misinterpreting all this.
>
> Moreover, having had a quick look at the lttng source code, I noticed
> it calls lttng-sessiond without the --daemonize option. I do not
> understand how the latter survives the former in such a case.
>
> Any idea on how I could further analyse this?
>
> Cheers
>
> -- Sebastien Barthelemy
>
>
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