linux-kernel - Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model

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Message-ID: <20160516172755.gro5enwei2gth3ko@treble>
Date:	Mon, 16 May 2016 12:27:55 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-s390@...r.kernel.org, Vojtech Pavlik <vojtech@...e.com>,
	Jiri Slaby <jslaby@...e.cz>, Petr Mladek <pmladek@...e.com>,
	Chris J Arges <chris.j.arges@...onical.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency
 model

On Mon, May 09, 2016 at 11:41:37AM +0200, Miroslav Benes wrote:
> > +void klp_init_transition(struct klp_patch *patch, int state)
> > +{
> > +	struct task_struct *g, *task;
> > +	unsigned int cpu;
> > +	struct klp_object *obj;
> > +	struct klp_func *func;
> > +	int initial_state = !state;
> > +
> > +	klp_transition_patch = patch;
> > +
> > +	/*
> > +	 * If the patch can be applied or reverted immediately, skip the
> > +	 * per-task transitions.
> > +	 */
> > +	if (patch->immediate)
> > +		return;
> > +
> > +	/*
> > +	 * Initialize all tasks to the initial patch state to prepare them for
> > +	 * switching to the target state.
> > +	 */
> > +	read_lock(&tasklist_lock);
> > +	for_each_process_thread(g, task)
> > +		task->patch_state = initial_state;
> > +	read_unlock(&tasklist_lock);
> > +
> > +	/*
> > +	 * Ditto for the idle "swapper" tasks.
> > +	 */
> > +	get_online_cpus();
> > +	for_each_online_cpu(cpu)
> > +		idle_task(cpu)->patch_state = initial_state;
> > +	put_online_cpus();
> > +
> > +	/*
> > +	 * Ensure klp_ftrace_handler() sees the task->patch_state updates
> > +	 * before the func->transition updates.  Otherwise it could read an
> > +	 * out-of-date task state and pick the wrong function.
> > +	 */
> > +	smp_wmb();
> > +
> > +	/*
> > +	 * Set the func transition states so klp_ftrace_handler() will know to
> > +	 * switch to the transition logic.
> > +	 *
> > +	 * When patching, the funcs aren't yet in the func_stack and will be
> > +	 * made visible to the ftrace handler shortly by the calls to
> > +	 * klp_patch_object().
> > +	 *
> > +	 * When unpatching, the funcs are already in the func_stack and so are
> > +	 * already visible to the ftrace handler.
> > +	 */
> > +	klp_for_each_object(patch, obj)
> > +		klp_for_each_func(obj, func)
> > +			func->transition = true;
> > +
> > +	/*
> > +	 * Set the global target patch state which tasks will switch to.  This
> > +	 * has no effect until the TIF_PATCH_PENDING flags get set later.
> > +	 */
> > +	klp_target_state = state;
> 
> I am afraid there is a problem for (patch->immediate == true) patches. 
> klp_target_state is not set for those and the comment is not entirely 
> true, because klp_target_state has an effect in several places.

Ah, you're right.  I moved this assignment here for v2.  It was
originally done before the patch->immediate check.  If I remember
correctly, I moved it closer to the barrier for better readability (but
I created a bug in the process).

> I guess we need to set klp_target_state even for immediate patches. Should 
> we also initialize it with KLP_UNDEFINED and set it to KLP_UNDEFINED in 
> klp_complete_transition()?

Yes, to both.

-- 
Josh