Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1645166 > unrolled thread

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

Back to article view | Back to linux.kernel

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  [PATCH 5/6] kmod: preempt on kmod_umh_threads_get() "Luis R. Rodriguez" <mcgrof@kernel.org> - 2017-05-19 05:30 +0200
    Re: [PATCH 5/6] kmod: preempt on kmod_umh_threads_get() Dmitry Torokhov <dmitry.torokhov@gmail.com> - 2017-05-20 00:30 +0200

#1645166 — [PATCH 5/6] kmod: preempt on kmod_umh_threads_get()

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)
-		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

[toc] | [next] | [standalone]

#1645984

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

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web