From patchwork Thu Feb 22 23:50:24 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: John Ogness X-Patchwork-Id: 10236621 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 15DBA6019D for ; Thu, 22 Feb 2018 23:53:20 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 0C4E528817 for ; Thu, 22 Feb 2018 23:53:20 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id F416928FB5; Thu, 22 Feb 2018 23:53:19 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 7CDBF28817 for ; Thu, 22 Feb 2018 23:53:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752218AbeBVXxD (ORCPT ); Thu, 22 Feb 2018 18:53:03 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:40107 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752047AbeBVXvN (ORCPT ); Thu, 22 Feb 2018 18:51:13 -0500 Received: from hsi-kbw-5-158-153-53.hsi19.kabel-badenwuerttemberg.de ([5.158.153.53] helo=linux.lab.linutronix.de.) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.80) (envelope-from ) id 1ep0aI-0002Br-AZ; Fri, 23 Feb 2018 00:47:30 +0100 From: John Ogness To: linux-fsdevel@vger.kernel.org Cc: Al Viro , Linus Torvalds , Christoph Hellwig , Thomas Gleixner , Peter Zijlstra , Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org Subject: [PATCH v2 5/6] fs/dcache: Avoid a try_lock loop in shrink_dentry_list() Date: Fri, 23 Feb 2018 00:50:24 +0100 Message-Id: <20180222235025.28662-6-john.ogness@linutronix.de> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20180222235025.28662-1-john.ogness@linutronix.de> References: <20180222235025.28662-1-john.ogness@linutronix.de> X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1, SHORTCIRCUIT=-0.0001 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP shrink_dentry_list() holds dentry->d_lock and needs to acquire dentry->d_inode->i_lock. This cannot be done with a spin_lock() operation because it's the reverse of the regular lock order. To avoid ABBA deadlocks it is done with a trylock loop. Trylock loops are problematic in two scenarios: 1) PREEMPT_RT converts spinlocks to 'sleeping' spinlocks, which are preemptible. As a consequence the i_lock holder can be preempted by a higher priority task. If that task executes the trylock loop it will do so forever and live lock. 2) In virtual machines trylock loops are problematic as well. The VCPU on which the i_lock holder runs can be scheduled out and a task on a different VCPU can loop for a whole time slice. In the worst case this can lead to starvation. Commits 47be61845c77 ("fs/dcache.c: avoid soft-lockup in dput()") and 046b961b45f9 ("shrink_dentry_list(): take parent's d_lock earlier") are addressing exactly those symptoms. Avoid the trylock loop by using dentry_kill(). When pruning ancestors, the same code applies that is used to kill a dentry in dput(). This also has the benefit that the locking order is now the same. First the inode is locked, then the parent. Signed-off-by: John Ogness --- fs/dcache.c | 22 ++-------------------- 1 file changed, 2 insertions(+), 20 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index 082361939b84..e470d49daa54 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -1130,26 +1130,8 @@ static void shrink_dentry_list(struct list_head *list) * fragmentation. */ dentry = parent; - while (dentry && !lockref_put_or_lock(&dentry->d_lockref)) { - parent = lock_parent(dentry); - if (dentry->d_lockref.count != 1) { - dentry->d_lockref.count--; - spin_unlock(&dentry->d_lock); - if (parent) - spin_unlock(&parent->d_lock); - break; - } - inode = dentry->d_inode; /* can't be NULL */ - if (unlikely(!spin_trylock(&inode->i_lock))) { - spin_unlock(&dentry->d_lock); - if (parent) - spin_unlock(&parent->d_lock); - cpu_relax(); - continue; - } - __dentry_kill(dentry); - dentry = parent; - } + while (dentry && !lockref_put_or_lock(&dentry->d_lockref)) + dentry = dentry_kill(dentry); } }