From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gregory Haskins Subject: Re: [RT PATCH v2] seqlock: serialize against writers Date: Fri, 29 Aug 2008 14:12:55 -0400 Message-ID: <48B83C27.3090908@gmail.com> References: <20080829154237.1196.66825.stgit@dev.haskins.net> <20080829180135.22450.54780.stgit@dev.haskins.net> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig1B2C7BEE8E0D67C0E25B9BD4" Cc: mingo@elte.hu, rostedt@goodmis.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, shemminger@vyatta.com To: andi@firstfloor.org Return-path: Received: from py-out-1112.google.com ([64.233.166.180]:40178 "EHLO py-out-1112.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753715AbYH2SPS (ORCPT ); Fri, 29 Aug 2008 14:15:18 -0400 Received: by py-out-1112.google.com with SMTP id p76so586910pyb.10 for ; Fri, 29 Aug 2008 11:15:18 -0700 (PDT) In-Reply-To: <20080829180135.22450.54780.stgit@dev.haskins.net> Sender: linux-rt-users-owner@vger.kernel.org List-ID: This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig1B2C7BEE8E0D67C0E25B9BD4 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Gregory Haskins wrote: > *Patch submitted for inclusion in PREEMPT_RT 26-rt4. Applies to 2.6.26= =2E3-rt3* > > Hi Ingo, Steven, Thomas, > Please consider for -rt4. This fixes a nasty deadlock on my systems = under > heavy load. > > [ > Changelog: > v2: only touch seqlock_t because raw_seqlock_t doesn't require > serialization and userspace cannot modify data during a read > > v1: initial release > ] > =20 Hi Andi, As it turns out, my distcc backend was an x86_64 machine running the v1 patch and I started to notice sometime today that certain cc1 jobs were sometimes (albeit rarely) segfaulting on me. I noticed that before I even published the first patch, but I chalked it up to a corrupt .o on my NFS home. Plus I was forgetting that the distcc machine was running the patch, and I would probably have never made the userspace connection had you not mentioned it. In any case, I self-built this v2 patch with v2 applied, and the segfaults have gone away. So I think we know several things: 1) You were right that this would cause an issue if the slow path is hit.= 2) Steven was right that userspace must use raw_seqlock_t because it no longer crashes with v2. 3) I am satisfied that my primary concern is still properly addressed. Hopefully everyone is satisfied with this patch now. Thanks for the help. It's much appreciated! -Greg --------------enig1B2C7BEE8E0D67C0E25B9BD4 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iEYEARECAAYFAki4PCgACgkQP5K2CMvXmqHiFgCggrJAb0vr4nQBTAuUIvPIh108 PlYAn0WHqPnuA0T/WSjNF5d5MurKyPFI =Gvsv -----END PGP SIGNATURE----- --------------enig1B2C7BEE8E0D67C0E25B9BD4--