Re: [PATCH master+7.12 v2 3/3] Add test for user context selection sync

Pedro Alves
2016-09-21T16:40:45Z

On 09/16/2016 03:01 AM, Simon Marchi wrote:
> On 2016-09-14 15:31, Pedro Alves wrote:
>> Hi Simon,
>>
>> I didn't try to understand everything in detail, but overall
>> it looks very nice now. Thank you very much. A few comments below.
>
> Oops, while trying to rerun the full testsuite, I noticed that it leaves
> "-ex \"set non-stop 1\"" in the GDBFLAGS... that's not good. You can
> see it easily if you swap the order of the "all-stop" and "non-stop"
> variations. Can you spot why the save_vars isn't working as intended?
> When I use a temporary variable instead, it works as intended...

Did you ever figure this out?

>> Did you try using barriers and breakpoints? Several tests use that
>> to make sure threads are past a point.
>
> I tried, but the issue is that depending on the scheduling, the threads
> might still be in the pthread_barrier_wait function when you stop.
>
> Consider this pseudo-code:
>
> thread_function:
> barrier_wait
> infinite_loop # This is where I want the threads to be stopped.
>
> main_thread ():
> initialize_barrier with n = 3
> spawn_thread (thread_function)
> spawn_thread (thread_function)
> barrier_wait
> breakpoint
>
> Once the main thread hits the breakpoint, we have the assurance that the
> threads have started, but we don't know where they are. They might not
> have exited the barrier_wait function, or they might be in the infinite
> loop. Adding a sleep before the breakpoint in the main thread is the
> only way to be reasonnably sure (assuming 1 second is enough...) that
> both threads will have reached the infinite loop.
>
> Actually, it might work by putting thread-specific breakpoints on the
> single-line infinite loop, then doing two "continue". This way I think
> we would be guaranteed that the two threads stop exactly at that line.
> With a regular breakpoint it might not work, since a thread could hit
> the breakpoint twice while the other still hasn't reached it.

What I've done in the past is something very much like your breakpoints
idea. Though, instead of a thread-specific breakpoint, I used a normal
breakpoint + "set scheduler-locking on" to control which thread got to
the breakpoint. See multiple-step-overs.exp, for example.
Using a thread-specific breakpoint maybe makes that a little simpler,
worth a try.

>> Hmm, looks like this version of the test still runs forever.
>
> I don't think so, the main thread sets the quit flag which unblocks the
> threads. If you run the executable you'll see it exits in ~2 seconds.

Ah, OK.

>> gdb_test_multiple ?
>
> Like this?
>
> set test "interrupt thread $inf.2"
>
> send_gdb "interrupt\n"
> gdb_test_multiple "" $test {
> -re "Thread.*2.*stopped" {
> pass $test
> }
> }

No need for the separate send_gdb call:

set test "interrupt thread $inf.2"
gdb_test_multiple "interrupt" $test {
-re "Thread.*2.*stopped" {
pass $test
}
}

>>> + with_spawn_id $gdb_main_spawn_id {
>>> + # TODO: it doesn't work as of now.
>>> + # match_re_or_ensure_not_output "$cli_re\r\n"
>>> "-thread-select, event on cli"
>>> + }
>>
>> Is there a plan here?
>
> I think that will go in the same basket as the fact that any MI command
> with --thread currently changes the selected thread silently (without
> any =thread-selected event). Currently, --thread changes the thread tot
> he desired one, then when the mi_cmd_thread_select tries to change the
> thread, it thinks that it was already the current thread, so that an
> event isn't necessary. This should get fixed in the next iteration,
> when we split the concepts of user-selected-ptid and
> internally-selected-ptid. Specifying --thread won't mess with the
> user-selected-ptid, but if you do "-thread-select --thread 2 2", then
> mi_cmd_thread_select will change the user-selected-ptid, generating an
> event.
>
> It's not pretty to leave it like this in the test though. Should I
> create a bug right now and kfail it? Leave it commented out but put a
> better description?

There's no right or wrong answer, but since you've already written the
bits, I'd be inclined to file bug and kfail.

Thanks,
Pedro Alves