From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from 74-93-104-97-Washington.hfc.comcastbusiness.net ([74.93.104.97]:60521 "EHLO sunset.davemloft.net" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751014AbXJWAVC (ORCPT ); Mon, 22 Oct 2007 20:21:02 -0400 Date: Mon, 22 Oct 2007 17:21:23 -0700 (PDT) Message-Id: <20071022.172123.78710569.davem@davemloft.net> Subject: Re: [PATCH 1/2] irq_flags_t: intro and core annotations From: David Miller In-Reply-To: <471D0C54.7060207@garzik.org> References: <20071022194714.GI27248@parisc-linux.org> <471D0C54.7060207@garzik.org> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org To: jeff@garzik.org Cc: akpm@linux-foundation.org, torvalds@linux-foundation.org, matthew@wil.cx, arnd@arndb.de, ralf@linux-mips.org, adobriyan@gmail.com, viro@ftp.linux.org.uk, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org List-ID: From: Jeff Garzik Date: Mon, 22 Oct 2007 16:47:16 -0400 > Let me add to the chorus of voices: I continually see two cases where > real bugs crop up: > > 1) hacker uses spin_lock_irq() in incorrect context (where it is not > safe to do a blind enable/disable) > > 2) hacker uses spin_lock_irq() correctly, but the surrounding code > changes, thus invalidating prior assumptions. > > I would even go so far as to support the drastic measure of deleting > spin_lock_irq(). > > spin_lock_irqsave() generates fewer bugs, is more future-proof, and by > virtue of 'flags' permits architectures a bit more flexibility. Whilst I agree with you fully on the error-prone'ness argument, reading the processor interrupt level to fill in the 'flags' can have non-trivial cost. I think it's about 9 cycles and a full pipeline flush on most sparc64 chips for example. The write to turn off interrupts costs about the same, so you'd essentially be doubling the execution cost in every case that you convert as you propose. I seem to recall that on modern x86 chips the cost of dorking around with eflags like this is even higher. What's the relative cost of pushfl/popl/pushl/popfl vs. cli/sti on like a core2 duo or a k8? For 64-bit powerpc's software interrupt disabling scheme it seems to cost is about equal for both cases.