Date: Mon, 5 Dec 2016 09:37:02 -0800
From: Davidlohr Bueso <dave@stgolabs.net>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>, mingo@kernel.org,
        john.stultz@linaro.org, dimitrysh@google.com,
        linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to
 not use wait-queues
Message-ID: <20161205173702.GA23348@linux-80c1.suse>
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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