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

Dmitry Torokhov
2017-05-19T22:27:18Z

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@kernel.org>
> ---
> 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