Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760520AbZAGU4v (ORCPT ); Wed, 7 Jan 2009 15:56:51 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754661AbZAGU4k (ORCPT ); Wed, 7 Jan 2009 15:56:40 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:60434 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753389AbZAGU4i (ORCPT ); Wed, 7 Jan 2009 15:56:38 -0500 Date: Wed, 7 Jan 2009 12:55:49 -0800 (PST) From: Linus Torvalds X-X-Sender: torvalds@localhost.localdomain To: Steven Rostedt cc: Peter Zijlstra , paulmck@linux.vnet.ibm.com, Gregory Haskins , Ingo Molnar , Matthew Wilcox , Andi Kleen , Chris Mason , Andrew Morton , Linux Kernel Mailing List , linux-fsdevel , linux-btrfs , Thomas Gleixner , Nick Piggin , Peter Morreale , Sven Dietrich Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning In-Reply-To: Message-ID: References: <87r63ljzox.fsf@basil.nowhere.org> <20090103191706.GA2002@parisc-linux.org> <1231093310.27690.5.camel@twins> <20090104184103.GE2002@parisc-linux.org> <1231242031.11687.97.camel@twins> <20090106121052.GA27232@elte.hu> <4963584A.4090805@novell.com> <20090106131643.GA15228@elte.hu> <1231248041.11687.107.camel@twins> <49636799.1010109@novell.com> <20090106214229.GD6741@linux.vnet.ibm.com> <1231278275.11687.111.camel@twins> <1231279660.11687.121.camel@twins> <1231281801.11687.125.camel@twins> <1231283778.11687.136.camel@twins> <1231329783.11687.287.camel@twins> <1231347442.11687.344.camel@twins> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) 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: 2207 Lines: 78 On Wed, 7 Jan 2009, Steven Rostedt wrote: > > Next comes the issue to know if the owner is still running. Wouldn't we > need to do something like > > if (task_thread_info(cpu_rq(cpu)->curr) == owner) Yes. After verifying that "cpu" is in a valid range. > I understand that this should not be a problem, but I'm afraid it will > give me nightmares at night. ;-) > > God that code had better be commented well. Well, the good news is that it really would be just a few - admittedly very subtle - lines, each basically generating just a couple of machine instructions. So we'd be looking at code where the actual assembly output should hopefully be in the ten-to-twenty instruction range, and the C code itself would be about five times as many comments as actual real lines. So the code really shouldn't be much worse than /* * Look out! "thread" is an entirely speculative pointer * access and not reliable. */ void loop_while_oncpu(struct mutex *lock, struct thread_struct *thread) { for (;;) { unsigned cpu; struct runqueue *rq; if (lock->owner != thread) break; /* * Need to access the cpu field knowing that * DEBUG_PAGEALLOC could have unmapped it if * the mutex owner just released it and exited. */ if (__get_user(cpu, &thread->cpu)) break; /* * Even if the access succeeded (likely case), * the cpu field may no longer be valid. FIXME: * this needs to validate that we can do a * get_cpu() and that we have the percpu area. */ if (cpu >= NR_CPUS) break; if (!cpu_online(cpu)) break; /* * Is that thread really running on that cpu? */ rq = cpu_rq(cpu); if (task_thread_info(rq->curr) != thread) break; cpu_relax(); } } and it all looks like it shouldn't be all that bad. Yeah, it's like 50 lines of C code, but it's mostly comments about subtle one-liners that really expand to almost no real code at all. Linus -- 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/