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: <20161205113212.GA31288@redhat.com>
Date:   Mon, 5 Dec 2016 12:32:13 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>, 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 12/05, Oleg Nesterov wrote:
>
> Yes, but on a second thought task_rcu_dereference() won't really help,

I forgot to explain why, see below.

> 	#define xxx_wait_event(xxx, event) {
> 		// comment to explain why
> 		WARN_ON(current->exit_state);

Otherwise this process/thread can be already (auto)reaped and wakeup
can't rely on rcu.

And task_rcu_dereference() can't help because it can return NULL in
this case.

Oleg.