diff mbox

[RFC] kernfs: create raw version kernfs_path_len and kernfs_path

Message ID 56D0B2AE.7020104@linaro.org
State New
Headers show

Commit Message

Yang Shi Feb. 26, 2016, 8:16 p.m. UTC 
On 2/26/2016 11:59 AM, Shi, Yang wrote:
> On 2/26/2016 11:37 AM, Shi, Yang wrote:

>> On 2/26/2016 10:56 AM, Steven Rostedt wrote:

>>> 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:

>>

>> 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.

>

> BTW, it sounds this is not the only point where kernfs_node could be

> updated, __kernfs_remove should need synchronize_sched too.


Should be synchronize_sched moved into kernfs_put itself, just before 
the kfree call since it is called at some places other than 
kernfs_remove and kernfs_rename_ns.



But, kernfs_put might be called recursively, is it fine?

Thanks,
Yang

>

> Thanks,

> Yang

>

>>

>> 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);

>>>

>>>

>>

>
diff mbox

Patch

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index a4b1db1..89c0c16 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -456,6 +456,12 @@  void kernfs_put(struct kernfs_node *kn)
         if (kernfs_type(kn) == KERNFS_LINK)
                 kernfs_put(kn->symlink.target_kn);

+       /*
+        * Tracepoints may be reading the old name. They are protected
+        * by rcu_read_lock_sched().
+        */
+       synchronize_sched();
+
         kfree_const(kn->name);

         if (kn->iattr) {