[lttng-dev] [PATCH lttng-tools] Fix: cleanup stream on snapshot failure

[Jonathan Rajotte-Julien]

Hi Marcel,

This is exactly the kind of reproducer we are looking for.

Thanks for providing it. I'll try it out and check if we need anything more in that patch.

Cheers

----- Original Message -----
> From: "Marcel Hamer" <marcel.hamer at windriver.com>
> To: "jonathan rajotte-julien" <jonathan.rajotte-julien at efficios.com>
> Cc: "lttng-dev" <lttng-dev at lists.lttng.org>
> Sent: Tuesday, May 31, 2022 7:28:55 AM
> Subject: Re: [lttng-dev] [PATCH lttng-tools] Fix: cleanup stream on snapshot failure

> Hello Jonathan,
> 
> On Mon, May 30, 2022 at 11:27:55AM -0400, Jonathan Rajotte-Julien wrote:
>> [Please note: This e-mail is from an EXTERNAL e-mail address]
>> 
>> Hi Marcel,
>> 
>> Thanks for sending this patch.
>> 
>> Looks sensible to me, still do you have a reproducer for it? I went back to bug
>> 1352 and even with https://bugs.lttng.org/attachments/546 was unable to force
>> the assert failure.
> 
> I can only reproduce it when running lttng-consumerd in a debugger
> environment, in my case gdb. My reproduction scenario is:
> 
> 1. Setting a breakpoint on snapshot_channel() inside
>   src/common/ust-consumer/ust-consumer.c
> 2. When the breakpoint hits, remove the the complete lttng directory
>   containing the session data.
> 3. Continue the lttng_consumerd process from gdb.
> 4. In that case you see a negative return value -1 from
>   consumer_stream_create_output_files() inside snapshot_channel().
> 5. Take another snapshot and you will see lttng_consumerd crash because
>   of the assert(!stream->trace_chunk); inside snapshot_channel(). This
>   last action does not require any breakpoint intervention.
> 
> The scenario seems to be very timing sensitive to reproduce. I do not
> have a clear command sequence to achieve the same error.
> 
> The proposed patch prevents lttng_consumerd from crashing in step 5.
> 
> Kind regards,
> 
> Marcel
> 
>> 
>> Cheers
>> 
>> ----- Original Message -----
>> > From: "Marcel Hamer via lttng-dev" <lttng-dev at lists.lttng.org>
>> > To: "lttng-dev" <lttng-dev at lists.lttng.org>
>> > Sent: Monday, 30 May, 2022 10:10:21
>> > Subject: [lttng-dev] [PATCH lttng-tools] Fix: cleanup stream on snapshot failure
>> 
>> > When a channel snapshot creation fails the stream should be cleaned up
>> > properly. If the stream is not closed and cleaned properly on a failure,
>> > the next time a snapshot is created an assert is triggered for:
>> >
>> >       assert(!stream->trace_chunk);
>> >
>> > inside the snapshot_channel function. Since the stream->trace_chunk was
>> > not reset to NULL. The reset to NULL happens inside the
>> > consumer_stream_close function.
>> >
>> > Fixes #1352
>> >
>> > Signed-off-by: Marcel Hamer <marcel.hamer at windriver.com>
>> > ---
>> > src/common/ust-consumer/ust-consumer.c | 10 +++++-----
>> > 1 file changed, 5 insertions(+), 5 deletions(-)
>> >
>> > diff --git a/src/common/ust-consumer/ust-consumer.c
>> > b/src/common/ust-consumer/ust-consumer.c
>> > index f176ca40a..f43216829 100644
>> > --- a/src/common/ust-consumer/ust-consumer.c
>> > +++ b/src/common/ust-consumer/ust-consumer.c
>> > @@ -1147,13 +1147,13 @@ static int snapshot_channel(struct
>> > lttng_consumer_channel *channel,
>> >               if (use_relayd) {
>> >                       ret = consumer_send_relayd_stream(stream, path);
>> >                       if (ret < 0) {
>> > -                             goto error_unlock;
>> > +                             goto error_close_stream;
>> >                       }
>> >               } else {
>> >                       ret = consumer_stream_create_output_files(stream,
>> >                                       false);
>> >                       if (ret < 0) {
>> > -                             goto error_unlock;
>> > +                             goto error_close_stream;
>> >                       }
>> >                       DBG("UST consumer snapshot stream (%" PRIu64 ")",
>> >                                       stream->key);
>> > @@ -1170,19 +1170,19 @@ static int snapshot_channel(struct
>> > lttng_consumer_channel *channel,
>> >               ret = lttng_ustconsumer_take_snapshot(stream);
>> >               if (ret < 0) {
>> >                       ERR("Taking UST snapshot");
>> > -                     goto error_unlock;
>> > +                     goto error_close_stream;
>> >               }
>> >
>> >               ret = lttng_ustconsumer_get_produced_snapshot(stream, &produced_pos);
>> >               if (ret < 0) {
>> >                       ERR("Produced UST snapshot position");
>> > -                     goto error_unlock;
>> > +                     goto error_close_stream;
>> >               }
>> >
>> >               ret = lttng_ustconsumer_get_consumed_snapshot(stream, &consumed_pos);
>> >               if (ret < 0) {
>> >                       ERR("Consumerd UST snapshot position");
>> > -                     goto error_unlock;
>> > +                     goto error_close_stream;
>> >               }
>> >
>> >               /*
>> > --
>> > 2.25.1
>> >
>> > _______________________________________________
>> > lttng-dev mailing list
>> > lttng-dev at lists.lttng.org
> > > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev