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

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Date:	Wed, 6 Apr 2016 15:06:19 +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 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);

		/*
		 * 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);
}


Best Regards,
Petr