From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754632Ab2KMDCl (ORCPT ); Mon, 12 Nov 2012 22:02:41 -0500 Received: from mail-da0-f46.google.com ([209.85.210.46]:36312 "EHLO mail-da0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754606Ab2KMDCh (ORCPT ); Mon, 12 Nov 2012 22:02:37 -0500 From: Tejun Heo To: lizefan@huawei.com, containers@lists.linux-foundation.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Cc: mhocko@suse.cz, glommer@parallels.com, Tejun Heo Subject: [PATCH 14/17] cgroup: use mutex_trylock() when grabbing i_mutex of a new cgroup directory Date: Mon, 12 Nov 2012 19:01:41 -0800 Message-Id: <1352775704-9023-15-git-send-email-tj@kernel.org> X-Mailer: git-send-email 1.7.11.7 In-Reply-To: <1352775704-9023-1-git-send-email-tj@kernel.org> References: <1352775704-9023-1-git-send-email-tj@kernel.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org All cgroup directory i_mutexes nest outside cgroup_mutex; however, new directory creation is a special case. A new cgroup directory is created while holding cgroup_mutex. Populating the new directory requires both the new directory's i_mutex and cgroup_mutex. Because all directory i_mutexes nest outside cgroup_mutex, grabbing both requires releasing cgroup_mutex first, which isn't a good idea as the new cgroup isn't yet ready to be manipulated by other cgroup opreations. This is worked around by grabbing the new directory's i_mutex while holding cgroup_mutex before making it visible. As there's no other user at that point, grabbing the i_mutex under cgroup_mutex can't lead to deadlock. cgroup_create_file() was using I_MUTEX_CHILD to tell lockdep not to worry about the reverse locking order; however, this creates pseudo locking dependency cgroup_mutex -> I_MUTEX_CHILD, which isn't true - all directory i_mutexes are still nested outside cgroup_mutex. This pseudo locking dependency can lead to spurious lockdep warnings. Use mutex_trylock() instead. This will always succeed and lockdep doesn't create any locking dependency for it. Signed-off-by: Tejun Heo --- kernel/cgroup.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index b42f63f..8ad5e76 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -2657,9 +2657,15 @@ static int cgroup_create_file(struct dentry *dentry, umode_t mode, inc_nlink(inode); inc_nlink(dentry->d_parent->d_inode); - /* start with the directory inode held, so that we can - * populate it without racing with another mkdir */ - mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD); + /* + * Control reaches here with cgroup_mutex held. + * @inode->i_mutex should nest outside cgroup_mutex but we + * want to populate it immediately without releasing + * cgroup_mutex. As @inode isn't visible to anyone else + * yet, trylock will always succeed without affecting + * lockdep checks. + */ + WARN_ON_ONCE(!mutex_trylock(&inode->i_mutex)); } else if (S_ISREG(mode)) { inode->i_size = 0; inode->i_fop = &cgroup_file_operations; -- 1.7.11.7