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

From: Ingo Molnar <mingo@elte.hu>
To: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Masami Hiramatsu <mhiramat@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Nick Piggin <npiggin@suse.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andi Kleen <andi@firstfloor.org>,
	linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Arjan van de Ven <arjan@infradead.org>,
	Rusty Russell <rusty@rustcorp.com.au>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Steven Rostedt <srostedt@redhat.com>
Subject: Re: [PATCH] Text Edit Lock - kprobes architecture independent support (v2)
Date: Tue, 3 Mar 2009 15:53:12 +0100	[thread overview]
Message-ID: <20090303145312.GA19483@elte.hu> (raw)
In-Reply-To: <20090303120659.GA3480@in.ibm.com>

* Ananth N Mavinakayanahalli <ananth@in.ibm.com> wrote:

> 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?

No, it's fine indeed, i got the locking order messed up ... 
twice :-)

	Ingo