linux-kernel - Re: [PATCH 1/3] jump_label: Pull get_online

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Date:   Fri, 21 Apr 2017 12:08:26 -0400
From:   Jason Baron <jbaron@...mai.com>
To:     Peter Zijlstra <peterz@...radead.org>, rostedt@...dmis.org,
        tglx@...utronix.de, mingo@...nel.org, bigeasy@...utronix.de
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] jump_label: Pull get_online_cpus() into generic code

On 04/18/2017 06:32 AM, Peter Zijlstra wrote:
> This change does two things; it moves the get_online_cpus() call into
> generic code, with the aim of later providing some static_key ops that
> avoid it.
>
> And as a side effect it inverts the relation between cpu_hotplug_lock
> and jump_label_mutex.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---

...

> @@ -146,6 +154,7 @@ static void __static_key_slow_dec(struct
>  	 * returns is unbalanced, because all other static_key_slow_inc()
>  	 * instances block while the update is in progress.
>  	 */
> +	get_online_cpus();
>  	if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) {
>  		WARN(atomic_read(&key->enabled) < 0,
>  		     "jump label: negative count!\n");

So the get and put can be unbalanced here since the above:

'if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex))'

is followed by 'return;'. However, I see that the next patch removes 
this and so things are balanced again...


> @@ -159,6 +168,7 @@ static void __static_key_slow_dec(struct
>  		jump_label_update(key);
>  	}
>  	jump_label_unlock();
> +	put_online_cpus();
>  }
>
>  static void jump_label_update_timeout(struct work_struct *work)
> @@ -592,6 +602,10 @@ jump_label_module_notify(struct notifier
>
>  	switch (val) {
>  	case MODULE_STATE_COMING:
> +		/*
> +		 * XXX do we need get_online_cpus() ?  the module isn't
> +		 * executable yet, so nothing should be looking at our code.
> +		 */

Since we're just updating the table of places we potentially need to 
patch, but not actually doing any patching, we should not need 
get_online_cpus() here...so in attempt to reduce confusion I would 
remove this.

Thanks,

-Jason