Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754798AbYCUDHc (ORCPT ); Thu, 20 Mar 2008 23:07:32 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753074AbYCUDHT (ORCPT ); Thu, 20 Mar 2008 23:07:19 -0400 Received: from netrider.rowland.org ([192.131.102.5]:4851 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1752651AbYCUDHR (ORCPT ); Thu, 20 Mar 2008 23:07:17 -0400 Date: Thu, 20 Mar 2008 23:07:16 -0400 (EDT) From: Alan Stern X-X-Sender: stern@netrider.rowland.org To: Andrew Morton cc: Michael Buesch , Henrique de Moraes Holschuh , David Brownell , Richard Purdie , , Ingo Molnar , Geert Uytterhoeven , , Martin Schwidefsky , Heiko Carstens , , , , Stefan Richter , Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c In-Reply-To: <20080320192719.6a32386e.akpm@linux-foundation.org> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1465 Lines: 33 On Thu, 20 Mar 2008, Andrew Morton wrote: > > > > Now, it happens that in_atomic() returns true on non-preemtible kernels > > > > when running in interrupt or softirq context. But if the above code really > > > > is using in_atomic() to detect am-i-called-from-interrupt and NOT > > > > am-i-called-from-inside-spinlock, they should be using in_irq(), > > > > in_softirq() or in_interrupt(). > > > > > > Presumably most of these places are actually trying to detect > > > am-i-allowed-to-sleep. Isn't that what in_atomic() is supposed to do? > > > > No, I think there is no such check in the kernel. Most likely for performance > > reasons, as it would require a global flag that is set on each spinlock. > > Yup. non-preemptible kernels avoid the inc/dec of > current_thread_info->preempt_count on spin_lock/spin_unlock So then what's the point of having in_atomic() at all? Is it nothing more than a shorthand form of (in_irq() | in_softirq() | in_interrupt())? In short, you are saying that there is _no_ reliable way to determine am-i-called-from-inside-spinlock. Well, why isn't there? Would it be so terrible if non-preemptible kernels did adjust preempt_count on spin_lock/unlock? Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/