From 27e89ae5d6e94e30231ee89a7736f62d84ba4c6f Mon Sep 17 00:00:00 2001 From: Li Zefan Date: Tue, 15 Jan 2013 14:10:57 +0800 Subject: [PATCH] cpuset: fix RCU lockdep splat 5d21cc2db040d01f8c19b8602f6987813e1176b4 ("cpuset: replace cgroup_mutex locking with cpuset internal locking") incorrectly converted proc_cpuset_show() from cgroup_lock() to cpuset_mutex. proc_cpuset_show() is accessing cgroup hierarchy proper to determine cgroup path which can't be protected by cpuset_mutex. This triggered the following RCU warning. =============================== [ INFO: suspicious RCU usage. ] 3.8.0-rc3-next-20130114-sasha-00016-ga107525-dirty #262 Tainted: G W ------------------------------- include/linux/cgroup.h:534 suspicious rcu_dereference_check() usage! other info that might help us debug this: rcu_scheduler_active = 1, debug_locks = 1 2 locks held by trinity/7514: #0: (&p->lock){+.+.+.}, at: [] seq_read+0x3a/0x3e0 #1: (cpuset_mutex){+.+...}, at: [] proc_cpuset_show+0x84/0x190 stack backtrace: Pid: 7514, comm: trinity Tainted: G W +3.8.0-rc3-next-20130114-sasha-00016-ga107525-dirty #262 Call Trace: [] lockdep_rcu_suspicious+0x10b/0x120 [] proc_cpuset_show+0x111/0x190 [] seq_read+0x1b7/0x3e0 [] ? seq_lseek+0x110/0x110 [] do_loop_readv_writev+0x4b/0x90 [] do_readv_writev+0xf6/0x1d0 [] vfs_readv+0x3e/0x60 [] sys_readv+0x50/0xd0 [] tracesys+0xe1/0xe6 The operation can be performed under RCU read lock. Replace cpuset_mutex locking with RCU read locking. tj: Rewrote patch description. Reported-by: Sasha Levin Signed-off-by: Li Zefan Signed-off-by: Tejun Heo --- kernel/cpuset.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/cpuset.c b/kernel/cpuset.c index 6aa5bbb..1a675e4 100644 --- a/kernel/cpuset.c +++ b/kernel/cpuset.c @@ -2678,15 +2678,15 @@ static int proc_cpuset_show(struct seq_file *m, void *unused_v) goto out_free; retval = -EINVAL; - mutex_lock(&cpuset_mutex); + rcu_read_lock(); css = task_subsys_state(tsk, cpuset_subsys_id); retval = cgroup_path(css->cgroup, buf, PAGE_SIZE); + rcu_read_unlock(); if (retval < 0) - goto out_unlock; + goto out_put_task; seq_puts(m, buf); seq_putc(m, '\n'); -out_unlock: - mutex_unlock(&cpuset_mutex); +out_put_task: put_task_struct(tsk); out_free: kfree(buf); -- 2.7.4