netdev - Re: [PATCH v4 1/3] make jump_labels wait while updates are in place

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Date:	Thu, 26 Apr 2012 20:43:06 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	cgroups@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
	Tejun Heo <tj@...nel.org>, kamezawa.hiroyu@...fujitsu.com,
	linux-mm@...ck.org, devel@...nvz.org,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, Ingo Molnar <mingo@...e.hu>,
	Jason Baron <jbaron@...hat.com>
Subject: Re: [PATCH v4 1/3] make jump_labels wait while updates are in place

On Thu, Apr 26, 2012 at 07:51:05PM -0300, Glauber Costa wrote:
> In mem cgroup, we need to guarantee that two concurrent updates
> of the jump_label interface wait for each other. IOW, we can't have
> other updates returning while the first one is still patching the
> kernel around, otherwise we'll race.

But it shouldn't. The code as is should prevent that.

> 
> I believe this is something that can fit well in the static branch
> API, without noticeable disadvantages:
> 
> * in the common case, it will be a quite simple lock/unlock operation
> * Every context that calls static_branch_slow* already expects to be
>   in sleeping context because it will mutex_lock the unlikely case.
> * static_key_slow_inc is not expected to be called in any fast path,
>   otherwise it would be expected to have quite a different name. Therefore
>   the mutex + atomic combination instead of just an atomic should not kill
>   us.
> 
> Signed-off-by: Glauber Costa <glommer@...allels.com>
> CC: Tejun Heo <tj@...nel.org>
> CC: Li Zefan <lizefan@...wei.com>
> CC: Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> CC: Johannes Weiner <hannes@...xchg.org>
> CC: Michal Hocko <mhocko@...e.cz>
> CC: Ingo Molnar <mingo@...e.hu>
> CC: Jason Baron <jbaron@...hat.com>
> ---
>  kernel/jump_label.c |   21 +++++++++++----------
>  1 files changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
> index 4304919..5d09cb4 100644
> --- a/kernel/jump_label.c
> +++ b/kernel/jump_label.c
> @@ -57,17 +57,16 @@ static void jump_label_update(struct static_key *key, int enable);
>  
>  void static_key_slow_inc(struct static_key *key)
>  {
> +	jump_label_lock();
>  	if (atomic_inc_not_zero(&key->enabled))
> -		return;

If key->enabled is not zero, there's nothing to be done. As the jump
label has already been enabled. Note, the key->enabled doesn't get set
until after the jump label is updated. Thus, if two tasks were to come
in, they both would be locked on the jump_label_lock().


> +		goto out;
>  
> -	jump_label_lock();
> -	if (atomic_read(&key->enabled) == 0) {
> -		if (!jump_label_get_branch_default(key))
> -			jump_label_update(key, JUMP_LABEL_ENABLE);
> -		else
> -			jump_label_update(key, JUMP_LABEL_DISABLE);
> -	}
> +	if (!jump_label_get_branch_default(key))
> +		jump_label_update(key, JUMP_LABEL_ENABLE);
> +	else
> +		jump_label_update(key, JUMP_LABEL_DISABLE);
>  	atomic_inc(&key->enabled);
> +out:
>  	jump_label_unlock();
>  }
>  EXPORT_SYMBOL_GPL(static_key_slow_inc);
> @@ -75,10 +74,11 @@ EXPORT_SYMBOL_GPL(static_key_slow_inc);
>  static void __static_key_slow_dec(struct static_key *key,
>  		unsigned long rate_limit, struct delayed_work *work)
>  {
> -	if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) {
> +	jump_label_lock();
> +	if (atomic_dec_and_test(&key->enabled)) {
>  		WARN(atomic_read(&key->enabled) < 0,
>  		     "jump label: negative count!\n");
> -		return;

Here, it is similar. If enabled is > 1, it wouldn't need to do anything,
thus it would dec the counter and return. But if it were one, then the
lock would be taken. and set to zero. There shouldn't be a case where
two tasks came in to set it less than zero (then something is
unbalanced).

Are you hitting the WARN_ON?

-- Steve

> +		goto out;
>  	}
>  
>  	if (rate_limit) {
> @@ -90,6 +90,7 @@ static void __static_key_slow_dec(struct static_key *key,
>  		else
>  			jump_label_update(key, JUMP_LABEL_ENABLE);
>  	}
> +out:
>  	jump_label_unlock();
>  }
>  
> -- 
> 1.7.7.6
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