Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934506Ab3CZO1K (ORCPT ); Tue, 26 Mar 2013 10:27:10 -0400 Received: from 173-166-109-252-newengland.hfc.comcastbusiness.net ([173.166.109.252]:35420 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759784Ab3CZO1H (ORCPT ); Tue, 26 Mar 2013 10:27:07 -0400 Message-ID: <1364308023.5053.40.camel@laptop> Subject: Re: [PATCH -mm -next] ipc,sem: fix lockdep false positive From: Peter Zijlstra To: Michel Lespinasse Cc: Rik van Riel , Sasha Levin , torvalds@linux-foundation.org, davidlohr.bueso@hp.com, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, hhuang@redhat.com, jason.low2@hp.com, lwoodman@redhat.com, chegu_vinod@hp.com, Dave Jones , benisty.e@gmail.com, Ingo Molnar Date: Tue, 26 Mar 2013 15:27:03 +0100 In-Reply-To: References: <1363809337-29718-1-git-send-email-riel@surriel.com> <5150B1C2.8090607@oracle.com> <20130325163844.042a45ba@annuminas.surriel.com> <1364303965.5053.29.camel@laptop> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.6.2-0ubuntu0.1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1661 Lines: 52 On Tue, 2013-03-26 at 06:40 -0700, Michel Lespinasse wrote: > sem_nsems is user provided as the array size in some semget system > call. It's the size of an ipc semaphore array. So we're basically adding a random (big) number to preempt_count (obviously while preemption is disabled), seems rather costly and undesirable. > complex semop operations take the array's lock plus every semaphore > locks; simple semop operations (operating on a single semaphore) only > take that one semaphore's lock. Right, standard global/local lock like stuff. Is there a way we can add a r/o test to the 'local' lock operation and avoid doing the above? Maybe something like: void sma_lock(struct sem_array *sma) /* global */ { int i; sma->global_locked = 1; smp_wmb(); /* can we merge with the LOCK ? */ spin_lock(&sma->global_lock); /* wait for all local locks to go away */ for (i = 0; i < sma->sem_nsems; i++) spin_unlock_wait(&sem->sem_base[i]->lock); } void sma_lock_one(struct sem_array *sma, int nr) /* local */ { smp_rmb(); /* pairs with wmb in sma_lock() */ if (unlikely(sma->global_locked)) { /* wait for global lock */ while (sma->global_locked) spin_unlock_wait(&sma->global_lock); } spin_lock(&sma->sem_base[nr]->lock); } This still has the problem of a non-preemptible section of O(sem_nsems) (with the avg wait-time on the local lock). Could we make the global lock a sleeping lock? -- 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/