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

[Steven Rostedt]

On Mon, 7 Mar 2016 11:41:37 -0500 (EST)
Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> 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