linux-kernel - Re: [PATCH] shm: optimize exit

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Date:	Wed, 3 Aug 2011 21:08:06 +0200
From:	Manuel Lauss <manuel.lauss@...glemail.com>
To:	Vasiliy Kulikov <segoon@...nwall.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Richard Weinberger <richard@....at>,
	Marc Zyngier <marc.zyngier@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] shm: optimize exit_shm()

Hello,

On Wed, Aug 3, 2011 at 8:28 PM, Vasiliy Kulikov <segoon@...nwall.com> wrote:
> We may check .in_use == 0 without holding the rw_mutex as .in_use is int
> and reads of ints are atomic.  As .in_use may be changed to zero while current
> process was sleeping in down_write(), we should check .in_use once again after
> down_write().
>
> Signed-off-by: Vasiliy Kulikov <segoon@...nwall.com>
> ---
>  ipc/shm.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
>
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 4e3c883..855ddc0 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -304,6 +304,9 @@ void exit_shm(struct task_struct *task)
>  {
>        struct ipc_namespace *ns = task->nsproxy->ipc_ns;
>
> +       if (shm_ids(ns).in_use == 0)
> +               return;
> +
>        /* Destroy all already created segments, but not mapped yet */
>        down_write(&shm_ids(ns).rw_mutex);
>        if (shm_ids(ns).in_use)

This check here is now unnecessary, yes?

And this also fixes the oops.

Manuel
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