linux-kernel - Re: epoll: possible bug from wakeup

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Date:	Sun, 10 Mar 2013 05:59:37 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Eric Wong <normalperson@...t.net>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	NeilBrown <neilb@...e.de>, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: epoll: possible bug from wakeup_source activation

On Sun, 2013-03-10 at 01:11 +0000, Eric Wong wrote:
>  
>  static void ep_destroy_wakeup_source(struct epitem *epi)
>  {
> -	wakeup_source_unregister(epi->ws);
> -	epi->ws = NULL;
> +	struct wakeup_source *ws = epi->ws;
> +
> +	rcu_assign_pointer(epi->ws, NULL);

There is no need to use a memory barrier before setting epi->ws to NULL

You should use RCU_POINTER_INIT()

> +	synchronize_rcu(); /* wait for ep_pm_stay_awake to finish */

Wont this add a significant slowdown ?

> +	wakeup_source_unregister(ws);
>  }

Please add the following in your .config

CONFIG_SPARSE_RCU_POINTER=y

and try :

make C=2 fs/eventpoll.o

And fix all warnings ;)



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