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: <20160407094700.GA27670@pathway.suse.cz>
Date:	Thu, 7 Apr 2016 11:47:00 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Josh Poimboeuf <jpoimboe@...hat.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 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. We could even move this check into the
livepatch code but then print_context_stack_reliable() will not
always give reliable results.

Best Regards,
Petr