Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760921AbXK1Plc (ORCPT ); Wed, 28 Nov 2007 10:41:32 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756615AbXK1PlY (ORCPT ); Wed, 28 Nov 2007 10:41:24 -0500 Received: from mtiwmhc11.worldnet.att.net ([204.127.131.115]:37531 "EHLO mtiwmhc11.worldnet.att.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754717AbXK1PlY (ORCPT ); Wed, 28 Nov 2007 10:41:24 -0500 Message-ID: <474D8C22.7010902@lwfinger.net> Date: Wed, 28 Nov 2007 09:41:22 -0600 From: Larry Finger User-Agent: Thunderbird 2.0.0.6 (X11/20070801) MIME-Version: 1.0 To: Andreas Schwab CC: LKML Subject: Re: Question regarding mutex locking References: <474CF06C.8020208@lwfinger.net> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 837 Lines: 25 Andreas Schwab wrote: > Larry Finger writes: > >> If a particular routine needs to lock a mutex, but it may be entered with that mutex already locked, >> would the following code be SMP safe? >> >> hold_lock = mutex_trylock() >> >> ... >> >> if (hold_lock) >> mutex_unlock() > > When two CPUs may enter the critical region at the same time, what is > the point of the mutex? Also, the first CPU may unlock the mutex while > the second one is still inside the critical region. Thank you for that answer. I think that I'm finally beginning to understand. Larry - 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/