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

["Michał Mirosław" <mirqus-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

2010/12/14 Mike Waychison <mikew-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>:
> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Acked-by: Matt Mackall <mpm-VDJrAJ4Gl5ZBDgjK7y7TUQ@public.gmane.org>
> ---
>  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