On 2/26/2016 10:56 AM, Steven Rostedt wrote:
On Fri, 26 Feb 2016 10:15:05 -0800 Yang Shi <yang.shi@xxxxxxxxxx> 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:
Thanks for pointing this out. Yes, it looks we need this since the tracepoints are protected by rcu_read_lock_sched.
Will add this in V2. Yang
-- 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);
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