From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753627AbcC3NP2 (ORCPT ); Wed, 30 Mar 2016 09:15:28 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:34798 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751425AbcC3NP0 (ORCPT ); Wed, 30 Mar 2016 09:15:26 -0400 Date: Wed, 30 Mar 2016 15:15:21 +0200 From: Peter Zijlstra To: Sedat Dilek Cc: Ingo Molnar , Alfredo Alvarez Fernandez , Linus Torvalds , "Theodore Ts'o" , linux-fsdevel , LKML Subject: Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260 Message-ID: <20160330131521.GE3430@twins.programming.kicks-ass.net> References: <20160327204810.GW6356@twins.programming.kicks-ass.net> <20160329084701.GA9393@gmail.com> <20160330093659.GS3408@twins.programming.kicks-ass.net> <20160330124356.GD3430@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 30, 2016 at 02:46:36PM +0200, Sedat Dilek wrote: > dependency chain hlocks: 49608 [max: 327680] OK, so that is still below the u16 limit, so you're seeing an actual hash collision and my patch will not cure that. A different hash function _might_ help, but eventually this is an unfixable problem. Our input space is (2^13)^48 = 2^(13*48) = 2^624 = ff'n huge, reducing that to 2^64 is bound to generate a collision at some point. [ technically the 48 held_lock spots are not fully independent, so (2^13)^48 is slightly overestimating it, but the numbers are big enough for this to not matter much. ]