From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gregory Haskins Subject: Re: [PATCH] seqlock: serialize against writers Date: Fri, 29 Aug 2008 12:35:25 -0400 Message-ID: <48B8254D.1010206@gmail.com> References: <20080829154237.1196.66825.stgit@dev.haskins.net> <87abevpzv7.fsf@basil.nowhere.org> <48B81F60.3080409@gmail.com> <20080829162216.GW26610@one.firstfloor.org> <48B82349.1020109@gmail.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigC537A9119F95D7DADBC68DF2" Cc: Andi Kleen , Gregory Haskins , mingo@elte.hu, tglx@linutronix.de, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org To: Steven Rostedt Return-path: Received: from mail-gx0-f16.google.com ([209.85.217.16]:42489 "EHLO mail-gx0-f16.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755271AbYH2Qhs (ORCPT ); Fri, 29 Aug 2008 12:37:48 -0400 Received: by gxk9 with SMTP id 9so5209890gxk.13 for ; Fri, 29 Aug 2008 09:37:47 -0700 (PDT) In-Reply-To: Sender: linux-rt-users-owner@vger.kernel.org List-ID: This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigC537A9119F95D7DADBC68DF2 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Steven Rostedt wrote: > On Fri, 29 Aug 2008, Gregory Haskins wrote: > > =20 >> Andi Kleen wrote: >> =20 >>>> Im running it on a x86_64 box as we speak. How can I tell if there = is a >>>> certain mode that is permitting this? >>>> =20 >>>> =20 >>> If the boot up says you're running with PMtimer then it uses the fall= back >>> (usually happens on pre Fam10h AMD boxes). A typical Intel box >>> would use the faster ring 3 only TSC path and then explode with your >>> change I bet.=20 >>> >>> Or step with gdb through gettimeofday() and see if it does a syscall.= >>> >>> -Andi >>> =20 >>> =20 >> It seems to be running fine with no indication it has fallen back.=20 >> Perhaps I need a certain workload to bring out the issue? >> =20 > > Perhaps you never hit the slow path in userland. That's the only place = it=20 > would write. Perhaps add a dummy static variable in the fast path, and = > write to it. See if that crashes you apps. > > -- Steve > =20 Yeah, ideas crossed in the mail ;) I could just force all of the seqbegins to hit the slowpath by hacking the code and see what happens (aside from slowing down, of course ;) Question: Which seqlock_t does userspace use? I assume it uses seqlock_t and not raw_seqlock_t. But the only reason that I ask is that I converted raw_seqlock_t to use the new style as well to be consistent, even though it is not strictly necessary for the same reasons. So if perchance userspace uses the raw variant, I could solve this issue by only re-working the seqlock_t variant. Kind of a long shot, but figured I would mention it :) -Greg --------------enigC537A9119F95D7DADBC68DF2 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 iEYEARECAAYFAki4JU0ACgkQP5K2CMvXmqERwQCfXTxV8DQlmO3Zbi58bk58lEFA 6AQAnAomtElSoo6OXBAIQwgMaZEhSNyl =FIv4 -----END PGP SIGNATURE----- --------------enigC537A9119F95D7DADBC68DF2--