linux-kernel - Re: [RFC PATCH v1.9 05/14] sched: horrible way to detect whether a task has been preempted

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Message-ID: <20160407143403.6f7wvjvh2r43e4la@treble.redhat.com>
Date:	Thu, 7 Apr 2016 09:34:03 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Jiri Kosina <jikos@...nel.org>, Jessica Yu <jeyu@...hat.com>,
	Miroslav Benes <mbenes@...e.cz>, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org, Vojtech Pavlik <vojtech@...e.com>
Subject: Re: [RFC PATCH v1.9 05/14] sched: horrible way to detect whether a
 task has been preempted

On Thu, Apr 07, 2016 at 11:47:00AM +0200, Petr Mladek wrote:
> On Wed 2016-04-06 11:33:56, Josh Poimboeuf wrote:
> > On Wed, Apr 06, 2016 at 03:06:19PM +0200, Petr Mladek wrote:
> > > On Fri 2016-03-25 14:34:52, Josh Poimboeuf wrote:
> > > > This is a horrible way to detect whether a task has been preempted.
> > > > Come up with something better: task flag?  or is there already an
> > > > existing mechanism?
> > > 
> > > What about using kallsyms_lookup_size_offset() to check the address.
> > > It is more heavyweight but less hacky. The following code seems
> > > to work for me:
> > > 
> > > bool in_preempt_schedule_irq(unsigned long addr)
> > > {
> > > 	static unsigned long size;
> > > 
> > > 	if (unlikely(!size)) {
> > > 		int ret;
> > > 
> > > 		ret = kallsyms_lookup_size_offset(
> > > 				(unsigned long)preempt_schedule_irq,
> > > 				size, NULL);
> 				^^^^
> It works even better with &size ;-)
> 
> > > 
> > > 		/*
> > > 		 * Warn when the function is used without kallsyms or
> > > 		 * when it is unable to locate preempt_schedule_irq().
> > > 		 * Be conservative and always return true in this case.
> > > 		 */
> > > 		if (WARN_ON(!ret))
> > > 			size = -1L;
> > > 	}
> > > 
> > > 	return (addr - (unsigned long)preempt_schedule_irq <= size);
> > > }
> > 
> > Yeah, that would definitely be better.  Though still somewhat hacky.
> 
> Yeah. Well this is the same approach that we use to check if a patched
> function is on the stack.

Oh, I agree that it's a good way to check if preempt_schedule_irq() is
on the stack.  I'm just not convinced that's the cleanest way to ask
"has this task been preempted".

> We could even move this check into the livepatch code but then
> print_context_stack_reliable() will not always give reliable results.

Why would moving the check to the livepatch code affect the reliability
of print_context_stack_reliable()?

-- 
Josh