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

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

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