linux-kernel - Re: [PATCH v3 02/22] netconsole: Introduce locking over the netpoll fields

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Date:	Wed, 15 Dec 2010 00:15:23 +0100
From:	Michał Mirosław <mirqus@...il.com>
To:	Mike Waychison <mikew@...gle.com>
Cc:	simon.kagstrom@...insight.net, davem@...emloft.net,
	nhorman@...driver.com, Matt Mackall <mpm@...enic.com>,
	adurbin@...gle.com, linux-kernel@...r.kernel.org,
	chavey@...gle.com, Greg KH <greg@...ah.com>,
	netdev@...r.kernel.org,
	Américo Wang <xiyou.wangcong@...il.com>,
	akpm@...ux-foundation.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v3 02/22] netconsole: Introduce locking over the netpoll fields

2010/12/14 Mike Waychison <mikew@...gle.com>:
> The netconsole driver currently doesn't do any locking over its
> configuration fields.  This can cause problems if we were to ever have
> concurrent writing to fields while somebody is enabling the service.
>
> For simplicity, this patch extends targets_list_lock to cover all
> configuration fields within the targets.  Macros are also added here to
> wrap accessors so that we check whether the target has been enabled with
> locking handled.
>
> Signed-off-by: Mike Waychison <mikew@...gle.com>
> Acked-by: Matt Mackall <mpm@...enic.com>
> ---
>  drivers/net/netconsole.c |  114 ++++++++++++++++++++++++++--------------------
>  1 files changed, 64 insertions(+), 50 deletions(-)
>
> diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
> index c87a49e..6e16888 100644
> --- a/drivers/net/netconsole.c
> +++ b/drivers/net/netconsole.c
> @@ -327,6 +327,7 @@ static ssize_t store_enabled(struct netconsole_target *nt,
>                             const char *buf,
>                             size_t count)
>  {
> +       unsigned long flags;
>        int err;
>        long enabled;
>
> @@ -335,6 +336,10 @@ static ssize_t store_enabled(struct netconsole_target *nt,
>                return enabled;
>
>        if (enabled) {  /* 1 */
> +               spin_lock_irqsave(&target_list_lock, flags);
> +               if (nt->enabled)
> +                       goto busy;
> +               spin_unlock_irqrestore(&target_list_lock, flags);
>

This looks wrong. Unless there is another lock or mutex covering this
function, at this point (after spin_unlock_irqrestore()) another
thread might set nt->enabled = 1.

Best Regards,
Michał Mirosław
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