Re: LinuxPPS & spinlocks

Satyam Sharma
2007-07-30T04:57:32Z

Hi Rodolfo,

On Sun, 29 Jul 2007, Rodolfo Giometti wrote:

> On Sat, Jul 28, 2007 at 05:11:17AM +0530, Satyam Sharma wrote:
>
> > Take the race between the time_pps_setparams() syscall and a concurrent
> > pps_event() from an interrupt for instance. From sys_time_pps_setparams,
> > the parameters for an existing source are not modified / set atomically,
> > which means a pps_event() called on the same source in between will see
> > invalid parameters ... and bad things will happen.
>
> I think this should be a good solution... :)
>
> diff --git a/drivers/pps/kapi.c b/drivers/pps/kapi.c
> index 08de71d..f0c42ec 100644
> --- a/drivers/pps/kapi.c
> +++ b/drivers/pps/kapi.c
> @@ -29,12 +29,6 @@
> #include <linux/pps.h>
>
> /*
> - * Local variables
> - */
> -
> -static spinlock_t pps_lock = SPIN_LOCK_UNLOCKED;
> -
> -/*
> * Local functions
> */
>
> diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
> index 9176c01..91b7e4d 100644
> --- a/drivers/pps/pps.c
> +++ b/drivers/pps/pps.c
> @@ -35,6 +35,7 @@
>
> struct pps_s pps_source[PPS_MAX_SOURCES];
> DEFINE_MUTEX(pps_mutex);
> +spinlock_t pps_lock = SPIN_LOCK_UNLOCKED;
>
> /*
> * Misc functions
> @@ -227,6 +228,8 @@ asmlinkage long sys_time_pps_setparams(int source,
> goto sys_time_pps_setparams_exit;
> }
>
> + spin_lock(&pps_lock);
> +
> /* Save the new parameters */
> ret = copy_from_user(&pps_source[source].params, params,
> sizeof(struct pps_kparams));
> @@ -245,6 +248,8 @@ asmlinkage long sys_time_pps_setparams(int source,
> pps_source[source].params.mode |= PPS_CANWAIT;
> pps_source[source].params.api_version = PPS_API_VERS;
>
> + spin_unlock(&pps_lock);
> +
> sys_time_pps_setparams_exit:
> mutex_unlock(&pps_mutex);

Nopes, this isn't quite correct/safe. I suggest you should read:

http://www.kernel.org/pub/linux/kernel/people/rusty/kernel-locking/

Satyam