Re: [RFC PATCH] kernfs: create raw version kernfs_path_len and kernfs_path

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 26 Feb 2016 10:15:05 -0800
Yang Shi <yang.shi@linaro.org> wrote:

> commit 5634cc2aa9aebc77bc862992e7805469dcf83dac ("writeback: update writeback
> tracepoints to report cgroup") made writeback tracepoints report cgroup
> writeback, but it may trigger the below bug on -rt kernel since kernfs_path
> and kernfs_path_len are called by tracepoints, which acquire sleeping lock.
> 
> BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:930
> in_atomic(): 1, irqs_disabled(): 0, pid: 625, name: kworker/u16:3
> INFO: lockdep is turned off.
> Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830
> 
> CPU: 7 PID: 625 Comm: kworker/u16:3 Not tainted 4.4.1-rt5 #20
> Hardware name: Freescale Layerscape 2085a RDB Board (DT)
> Workqueue: writeback wb_workfn (flush-7:0)
> Call trace:
> [<ffffffc00008d708>] dump_backtrace+0x0/0x200
> [<ffffffc00008d92c>] show_stack+0x24/0x30
> [<ffffffc0007b0f40>] dump_stack+0x88/0xa8
> [<ffffffc000127d74>] ___might_sleep+0x2ec/0x300
> [<ffffffc000d5d550>] rt_spin_lock+0x38/0xb8
> [<ffffffc0003e0548>] kernfs_path_len+0x30/0x90
> [<ffffffc00036b360>] trace_event_raw_event_writeback_work_class+0xe8/0x2e8
> [<ffffffc000374f90>] wb_writeback+0x620/0x830
> [<ffffffc000376224>] wb_workfn+0x61c/0x950
> [<ffffffc000110adc>] process_one_work+0x3ac/0xb30
> [<ffffffc0001112fc>] worker_thread+0x9c/0x7a8
> [<ffffffc00011a9e8>] kthread+0x190/0x1b0
> [<ffffffc000086ca0>] ret_from_fork+0x10/0x30
> 
> Since kernfs_* functions are heavily used by cgroup, so it sounds not
> reasonable to convert kernfs_rename_lock to raw lock.
> 
> Create raw version kernfs_path, kernfs_path_len and cgroup_path, which don't
> acquire lock and are used by tracepoints only.
> 

And what prevents name from being freed while the tracepoint is reading
it?

Perhaps we need this change as well:

-- Steve

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 996b7742c90b..d2ef153145c0 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1397,6 +1397,12 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
 	kn->hash = kernfs_name_hash(kn->name, kn->ns);
 	kernfs_link_sibling(kn);
 
+	/*
+	 * Tracepoints may be reading the old name. They are protected
+	 * by rcu_read_lock_sched().
+	 */
+	synchronize_sched();
+
 	kernfs_put(old_parent);
 	kfree_const(old_name);