Re: [PATCH 1/3] jump_label: Pull get_online_cpus() into generic code

[Jason Baron]

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) <pet...@infradead.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