linux-kernel - Re: [PATCH] usbhid: Fix lockdep unannotated irqs-off warning

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Date:	Mon, 7 Mar 2016 12:03:18 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Sedat Dilek <sedat.dilek@...il.com>,
	Jiri Kosina <jikos@...nel.org>, Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Andy Lutomirski <luto@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	USB list <linux-usb@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] usbhid: Fix lockdep unannotated irqs-off warning

On Mon, 7 Mar 2016 11:41:37 -0500 (EST)
Alan Stern <stern@...land.harvard.edu> wrote:

> It's hard to call this a compiler bug, but perhaps it is -- I don't
> know how programmers are supposed to tell CLANG that a subroutine
> modifies the Interrupt Flag in a way that the compiler shouldn't mess
> up.


Really! This is what's is happening??

Clang takes this:

	if (!--hid->open) {
		spin_unlock_irq(X);
		do_something();
	} else {
		spin_unlock_irq(X);
	}

Thus it's basically doing:

	FLAG = !--hid->open;
	push flags;
	spin_unlock_irq(X)
	pop flags;
	if (FLAG zero set) {
		do_something();
	}


OUCH!!! There's gotta be a way to turn that off, otherwise Clang can
not be used to compile the kernel.

Nice detective work.

-- Steve