Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to not use wait-queues

From	Davidlohr Bueso <dave@stgolabs.net>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to not use wait-queues
Date	2016-12-05 18:40 +0100
Message-ID	<sL5p8-7ud-19@gated-at.bofh.it> (permalink)
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On Mon, 05 Dec 2016, Oleg Nesterov wrote:

>Yes. But percpu_down_write() should not be used after exit_notify(), so we
>can rely on rcu_read_lock(), release_task()->call_rcu(delayed_put_task_struct)
>can't be called until an exiting task passes exit_notify().
>
>But then we probably need WARN_ON(current->exit_state) in percpu_down_write().

Hmm, my immediate thought would have been doing a PF_EXITING check, but of course
this enlarges the window of the warn being triggered, yet maintains what you are
saying in that percpu_down_write should not be used after do_exit/exit_notify.
Furthermore, reading the comment in task_rcu_dereference, I get your point and
we can loose the reference to the task, iow busted rcu read cr.

>
>And personally I think this change should add the new helpers, they can have
>more users. Something like
>
>	struct xxx {

What do you think of s/xxx/rcuwait?

Thanks,
Davidlohr

Thread

[PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to not  use wait-queues Davidlohr Bueso <dave@stgolabs.net> - 2016-12-03 03:20 +0100
  Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to  not use wait-queues Peter Zijlstra <peterz@infradead.org> - 2016-12-05 09:40 +0100
    Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake         to not use wait-queues Oleg Nesterov <oleg@redhat.com> - 2016-12-05 12:30 +0100
      Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake         to not use wait-queues Oleg Nesterov <oleg@redhat.com> - 2016-12-05 12:50 +0100
      Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to  not use wait-queues Davidlohr Bueso <dave@stgolabs.net> - 2016-12-05 18:40 +0100
    Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake         to not use wait-queues Oleg Nesterov <oleg@redhat.com> - 2016-12-05 18:20 +0100
  Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake         to not use wait-queues Oleg Nesterov <oleg@redhat.com> - 2016-12-05 18:20 +0100

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