Date: Mon, 7 Mar 2016 12:03:18 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Sedat Dilek <sedat.dilek@gmail.com>, Jiri Kosina <jikos@kernel.org>,
        Tejun Heo <tj@kernel.org>, Lai Jiangshan <jiangshanlai@gmail.com>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Andy Lutomirski <luto@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH] usbhid: Fix lockdep unannotated irqs-off warning
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On Mon, 7 Mar 2016 11:41:37 -0500 (EST)
Alan Stern <stern@rowland.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