From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S264494AbTK3BAb (ORCPT ); Sat, 29 Nov 2003 20:00:31 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S264543AbTK3BAb (ORCPT ); Sat, 29 Nov 2003 20:00:31 -0500 Received: from h008.c007.snv.cp.net ([209.228.33.236]:20160 "HELO c007.snv.cp.net") by vger.kernel.org with SMTP id S264494AbTK3BAa (ORCPT ); Sat, 29 Nov 2003 20:00:30 -0500 X-Sent: 30 Nov 2003 01:00:29 GMT Message-ID: <000d01c3b6dd$30ab34a0$8a04a943@bananacabana> From: "Chris Peterson" To: Subject: question about preempt_disable() Date: Sat, 29 Nov 2003 16:59:21 -0800 MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2800.1158 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1165 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org I just bought Robert Love's new book "Linux Kernel Development". The book has been very informative, but I have some unanswered questions about kernel preemption. >>From what I understand, SMP-safe code is also preempt-safe. The preempt count is the number of spinlocks held by the current kernel thread. If the preempt code is greater zero, then the kernel thread cannot be preempted. My question is: if the code is already SMP-safe and holding the necessary spinlocks, why is the preempt count necessary? Why must preemption be disabled and re-enabled as spinlocks are acquired and released? Is it just an optimization for accessing per-cpu data? Or is it necessary to prevent priority inversion of kernel threads, when a low priority thread holds spinlock X and is preempted by a high priority thread that hogs the CPU, forever spinning in spin_lock(&X)? chris