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    SubjectRe: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for signaled process.
    On Tue, Apr 24, 2018 at 05:37:08PM -0400, Andrey Grodzovsky wrote:
    >
    >
    > On 04/24/2018 05:21 PM, Eric W. Biederman wrote:
    > > Andrey Grodzovsky <Andrey.Grodzovsky@amd.com> writes:
    > >
    > > > On 04/24/2018 03:44 PM, Daniel Vetter wrote:
    > > > > On Tue, Apr 24, 2018 at 05:46:52PM +0200, Michel Dänzer wrote:
    > > > > > Adding the dri-devel list, since this is driver independent code.
    > > > > >
    > > > > >
    > > > > > On 2018-04-24 05:30 PM, Andrey Grodzovsky wrote:
    > > > > > > Avoid calling wait_event_killable when you are possibly being called
    > > > > > > from get_signal routine since in that case you end up in a deadlock
    > > > > > > where you are alreay blocked in singla processing any trying to wait
    > > > > > Multiple typos here, "[...] already blocked in signal processing and [...]"?
    > > > > >
    > > > > >
    > > > > > > on a new signal.
    > > > > > >
    > > > > > > Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
    > > > > > > ---
    > > > > > > drivers/gpu/drm/scheduler/gpu_scheduler.c | 5 +++--
    > > > > > > 1 file changed, 3 insertions(+), 2 deletions(-)
    > > > > > >
    > > > > > > diff --git a/drivers/gpu/drm/scheduler/gpu_scheduler.c b/drivers/gpu/drm/scheduler/gpu_scheduler.c
    > > > > > > index 088ff2b..09fd258 100644
    > > > > > > --- a/drivers/gpu/drm/scheduler/gpu_scheduler.c
    > > > > > > +++ b/drivers/gpu/drm/scheduler/gpu_scheduler.c
    > > > > > > @@ -227,9 +227,10 @@ void drm_sched_entity_do_release(struct drm_gpu_scheduler *sched,
    > > > > > > return;
    > > > > > > /**
    > > > > > > * The client will not queue more IBs during this fini, consume existing
    > > > > > > - * queued IBs or discard them on SIGKILL
    > > > > > > + * queued IBs or discard them when in death signal state since
    > > > > > > + * wait_event_killable can't receive signals in that state.
    > > > > > > */
    > > > > > > - if ((current->flags & PF_SIGNALED) && current->exit_code == SIGKILL)
    > > > > > > + if (current->flags & PF_SIGNALED)
    > > > > You want fatal_signal_pending() here, instead of inventing your own broken
    > > > > version.
    > > > I rely on current->flags & PF_SIGNALED because this being set from
    > > > within get_signal,
    > > It doesn't mean that. Unless you are called by do_coredump (you
    > > aren't).
    >
    > Looking in latest code here
    > https://elixir.bootlin.com/linux/v4.17-rc2/source/kernel/signal.c#L2449
    > i see that current->flags |= PF_SIGNALED; is out side of
    > if (sig_kernel_coredump(signr)) {...} scope

    Ok I read some more about this, and I guess you go through process exit
    and then eventually close. But I'm not sure.

    The code in drm_sched_entity_fini also looks strange: You unpark the
    scheduler thread before you remove all the IBs. At least from the comment
    that doesn't sound like what you want to do.

    But in general, PF_SIGNALED is really something deeply internal to the
    core (used for some book-keeping and accounting). The drm scheduler is the
    only thing looking at it, so smells like a layering violation. I suspect
    (but without knowing what you're actually trying to achive here can't be
    sure) you want to look at something else.

    E.g. PF_EXITING seems to be used in a lot more places to cancel stuff
    that's no longer relevant when a task exits, not PF_SIGNALED. There's the
    TIF_MEMDIE flag if you're hacking around issues with the oom-killer.

    This here on the other hand looks really fragile, and probably only does
    what you want to do by accident.
    -Daniel

    >
    > Andrey
    >
    > > The closing of files does not happen in do_coredump.
    > > Which means you are being called from do_exit.
    > > In fact you are being called after exit_files which closes
    > > the files. The actual __fput processing happens in task_work_run.
    > >
    > > > meaning I am within signal processing  in which case I want to avoid
    > > > any signal based wait for that task,
    > > > From what i see in the code, task_struct.pending.signal is being set
    > > > for other threads in same
    > > > group (zap_other_threads) or for other scenarios, those task are still
    > > > able to receive signals
    > > > so calling wait_event_killable there will not have problem.
    > > Excpet that you are geing called after from do_exit and after exit_files
    > > which is after exit_signal. Which means that PF_EXITING has been set.
    > > Which implies that the kernel signal handling machinery has already
    > > started being torn down.
    > >
    > > Not as much as I would like to happen at that point as we are still
    > > left with some old CLONE_PTHREAD messes in the code that need to be
    > > cleaned up.
    > >
    > > Still given the fact you are task_work_run it is quite possible even
    > > release_task has been run on that task before the f_op->release method
    > > is called. So you simply can not count on signals working.
    > >
    > > Which in practice leaves a timeout for ending your wait. That code can
    > > legitimately be in a context that is neither interruptible nor killable.
    > >
    > > > > > > entity->fini_status = -ERESTARTSYS;
    > > > > > > else
    > > > > > > entity->fini_status = wait_event_killable(sched->job_scheduled,
    > > > > But really this smells like a bug in wait_event_killable, since
    > > > > wait_event_interruptible does not suffer from the same bug. It will return
    > > > > immediately when there's a signal pending.
    > > > Even when wait_event_interruptible is called as following -
    > > > ...->do_signal->get_signal->....->wait_event_interruptible ?
    > > > I haven't tried it but wait_event_interruptible is very much alike to
    > > > wait_event_killable so I would assume it will also
    > > > not be interrupted if called like that. (Will give it a try just out
    > > > of curiosity anyway)
    > > As PF_EXITING is set want_signal should fail and the signal state of the
    > > task should not be updatable by signals.
    > >
    > > Eric
    > >
    > >
    >
    > _______________________________________________
    > dri-devel mailing list
    > dri-devel@lists.freedesktop.org
    > https://lists.freedesktop.org/mailman/listinfo/dri-devel

    --
    Daniel Vetter
    Software Engineer, Intel Corporation
    http://blog.ffwll.ch

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