Re: [PATCH] Text Edit Lock - kprobes architecture independent support (v2)

Ananth N Mavinakayanahalli
2009-03-03T12:07:13Z

On Tue, Mar 03, 2009 at 10:27:50AM +0100, Ingo Molnar wrote:
>
> * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
>
> > @@ -709,7 +711,8 @@ int __kprobes register_kprobe(struct kpr

Hi Ingo,

> > if (kprobe_enabled)
> > arch_arm_kprobe(p);
>
> hm, it's cleaner now, but there's serious locking dependency
> problems visible in the patch:
>
> > -
> > +out_unlock_text:
> > + mutex_unlock(&text_mutex);
> > out:
> > mutex_unlock(&kprobe_mutex);
>
> this one creates a (text_mutex -> kprobe_mutex) dependency.
> (also you removed a newline spuriously - dont do that)

That is a mutex_unlock :-) ...

> > @@ -746,8 +749,11 @@ valid_p:
> > * enabled and not gone - otherwise, the breakpoint would
> > * already have been removed. We save on flushing icache.
> > */
> > - if (kprobe_enabled && !kprobe_gone(old_p))
> > + if (kprobe_enabled && !kprobe_gone(old_p)) {
> > + mutex_lock(&text_mutex);
> > arch_disarm_kprobe(p);
> > + mutex_unlock(&text_mutex);
> > + }
> > hlist_del_rcu(&old_p->hlist);
>
> (kprobe_mutex -> text_mutex) dependency. AB-BA deadlock.

At this time the kprobe_mutex is already held.

...

> > @@ -1280,12 +1285,14 @@ static void __kprobes enable_all_kprobes
> > if (kprobe_enabled)
> > goto already_enabled;
> >
> > + mutex_lock(&text_mutex);
> > for (i = 0; i < KPROBE_TABLE_SIZE; i++) {
> > head = &kprobe_table[i];
> > hlist_for_each_entry_rcu(p, node, head, hlist)
> > if (!kprobe_gone(p))
> > arch_arm_kprobe(p);
> > }
> > + mutex_unlock(&text_mutex);
>
> this one creates a (kprobe_mutex -> text_mutex) dependency
> again.

kprobe_mutex his held here too...

> > @@ -1310,6 +1317,7 @@ static void __kprobes disable_all_kprobe
> >
> > kprobe_enabled = false;
> > printk(KERN_INFO "Kprobes globally disabled\n");
> > + mutex_lock(&text_mutex);
> > for (i = 0; i < KPROBE_TABLE_SIZE; i++) {
> > head = &kprobe_table[i];
> > hlist_for_each_entry_rcu(p, node, head, hlist) {
> > @@ -1317,7 +1325,7 @@ static void __kprobes disable_all_kprobe
> > arch_disarm_kprobe(p);
> > }
> > }
> > -
> > + mutex_unlock(&text_mutex);
> > mutex_unlock(&kprobe_mutex);
>
> And this one in the reverse direction again.

Unlock again :-)

> The dependencies are totally wrong. The text lock (a low level
> lock) should nest inside the kprobes mutex (which is the higher
> level lock).

>From what I see, Mathieu has done just that and has gotten the order
right in all cases. Or maybe I am missing something?

(I recall having tested this patch with LOCKDEP turned on and it
din't throw any errors).

Ananth