From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:50502 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726890AbeHJAjp (ORCPT ); Thu, 9 Aug 2018 20:39:45 -0400 From: NeilBrown To: "J. Bruce Fields" Date: Fri, 10 Aug 2018 08:12:43 +1000 Cc: Jeff Layton , Alexander Viro , Martin Wilck , linux-fsdevel@vger.kernel.org, Frank Filz , linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups In-Reply-To: <20180809173245.GM23873@fieldses.org> References: <153378012255.1220.6754153662007899557.stgit@noble> <20180809173245.GM23873@fieldses.org> Message-ID: <87lg9frxyc.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Sender: linux-fsdevel-owner@vger.kernel.org List-ID: --=-=-= Content-Type: text/plain On Thu, Aug 09 2018, J. Bruce Fields wrote: > I think there's also a problem with multiple tasks sharing the same > lock owner. > > So, all locks are exclusive locks for the same range. We have four > tasks. Tasks 1 and 4 share the same owner, the others' owners are > distinct. > > - Task 1 gets a lock. > - Task 2 gets a conflicting lock. > - Task 3 gets another conflicting lock. So now we the tree is > 3->2->1. > - Task 1's lock is released. > - Before task 2 is scheduled, task 4 acquires a new lock. > - Task 2 waits on task 4's lock, we now have > 3->2->4. > > Task 3 shouldn't be waiting--the lock it's requesting has the same owner > as the lock task 4 holds--but we fail to wake up task 3. So task 1 and task 4 are threads in the one process - OK. Tasks 2 and 3 are threads in two other processes. So 2 and 3 conflict with either 1 or 4 equally - why should task 3 be woken? I suspect you got the numbers bit mixed up, but in any case, the "conflict()" function that is passed around takes ownership into account when assessing if one lock conflicts with another. NeilBrown --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAltsvFsACgkQOeye3VZi gbnd5g/9HP79msN6KOY9iUVDlp6PPFKF9RC0kvf4INOz05YM/AxZ6EccXZxg/jdq WagfSxoKDD2qPf1WjoxzgMI0WTkbvNi/1YiOEC41HfGWZbMtwRCNjNcg+rw3pmLz sTvyjsstGalxLVu1nEmdYLGt0RieTl6WzcUIEoWH3IxIqSqkyQRxJ/VFwlk+JYVX t9N9uMyrGsEEEvMWEZ+IDK6RGodHwKbmSmpCmVYTPaqGYZ6Dd9pz+ZSSdNgwp9qQ FjlkFBIMvKWzXJDa1mmZpaWZcXylCgBxCAelGsd8AMtxGApABtuJbezwjpL2BbXK x/UrsBeIYU3EuF9TBa52vrq4eTo8OKyXxJl66R2YPsPwmgkDLlfep0ToBZcPbhkl vF1fhB+tfXJ297wblupWOHKDuNDA8Vl++Fmjhql7s174yN4Ak6W8gD6Tmgrsb5Yl 3gnzRh+9+Q7muFIsKD+QEo7dSa9Lk4TV0xYN3COHD3ey2tNXzkhl3a/WSk/CpmMh QnlQqypsKyHA8T4WxqOLE70K+wBsxdp5Ct0gG8Mm12ApO+IETidp48SJNGRicZul 90OcUHXhx8SieIQkJrb1hOwAhp0vmk2oZlZT0eO1f31yJq9OaU+SkR7+qCHgl/Yd Hnfgei/bnHlTdA9/YOqPjjX/2MqV+lfHmu3wWBErz2Ot5mwrkMQ= =Ei+X -----END PGP SIGNATURE----- --=-=-=--