Re: [PATCH 5/6] kmod: preempt on kmod_umh_threads_get()

[Dmitry Torokhov]

On Thu, May 18, 2017 at 08:24:43PM -0700, Luis R. Rodriguez wrote:
> In theory it is possible multiple concurrent threads will try to
> kmod_umh_threads_get() and as such atomic_inc(&kmod_concurrent) at
> the same time, therefore enabling a small time during which we've
> bumped kmod_concurrent but have not really enabled work. By using
> preemption we mitigate this a bit.
> 
> Preemption is not needed when we kmod_umh_threads_put().
> 
> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> ---
>  kernel/kmod.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index 563600fc9bb1..7ea11dbc7564 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -113,15 +113,35 @@ static int call_modprobe(char *module_name, int wait)
>  
>  static int kmod_umh_threads_get(void)
>  {
> +	int ret = 0;
> +
> +	/*
> +	 * Disabling preemption makes sure that we are not rescheduled here
> +	 *
> +	 * Also preemption helps kmod_concurrent is not increased by mistake
> +	 * for too long given in theory two concurrent threads could race on
> +	 * atomic_inc() before we atomic_read() -- we know that's possible
> +	 * and but we don't care, this is not used for object accounting and
> +	 * is just a subjective threshold. The alternative is a lock.
> +	 */
> +	preempt_disable();
>  	atomic_inc(&kmod_concurrent);
>  	if (atomic_read(&kmod_concurrent) <= max_modprobes)

That is very "fancy" way to basically say:

	if (atomic_inc_return(&kmod_concurrent) <= max_modprobes)
		...

> -		return 0;
> +		goto out;
> +
>  	atomic_dec(&kmod_concurrent);
> -	return -EBUSY;
> +	ret = -EBUSY;
> +out:
> +	preempt_enable();
> +	return ret;
>  }
>  
>  static void kmod_umh_threads_put(void)
>  {
> +	/*
> +	 * Preemption is not needed given once work is done we can
> +	 * pace ourselves on our way out.
> +	 */
>  	atomic_dec(&kmod_concurrent);
>  }
>  
> -- 
> 2.11.0
> 

Thanks.

-- 
Dmitry