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SubjectRe: [PATCH v12 00/12]
On Thu 2018-08-30 13:58:15, Miroslav Benes wrote:
> On Tue, 28 Aug 2018, Petr Mladek wrote:
>
> > livepatch: Atomic replace feature
> >
> > The atomic replace allows to create cumulative patches. They
> > are useful when you maintain many livepatches and want to remove
> > one that is lower on the stack. In addition it is very useful when
> > more patches touch the same function and there are dependencies
> > between them.
> >
> > This version does another big refactoring based on feedback against
> > v11[*]. In particular, it removes the registration step, changes
> > the API and handling of livepatch dependencies. The aim is
> > to keep the number of possible variants on a sane level.
> > It helps the keep the feature "easy" to use and maintain.
> >
> > [*] https://lkml.kernel.org/r/20180323120028.31451-1-pmladek@suse.com
>
> Hi,
>
> I've started to review the patch set. Running selftests with lockdep
> enabled gives me...
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 4.17.0-rc1-klp_replace_v12-117114-gfedb3eba611d #218 Tainted: G
> K
> ------------------------------------------------------
> kworker/1:1/49 is trying to acquire lock:
> 00000000bb88dc17 (kn->count#186){++++}, at: kernfs_remove+0x23/0x40
>
> but task is already holding lock:
> 0000000073632424 (klp_mutex){+.+.}, at: klp_transition_work_fn+0x17/0x40
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #1 (klp_mutex){+.+.}:
> lock_acquire+0xd4/0x220
> __mutex_lock+0x75/0x920
> mutex_lock_nested+0x1b/0x20
> enabled_store+0x47/0x150
> kobj_attr_store+0x12/0x20
> sysfs_kf_write+0x4a/0x60
> kernfs_fop_write+0x123/0x1b0
> __vfs_write+0x2b/0x150
> vfs_write+0xc7/0x1c0
> ksys_write+0x49/0xa0
> __x64_sys_write+0x1a/0x20
> do_syscall_64+0x62/0x1b0
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> -> #0 (kn->count#186){++++}:
> __lock_acquire+0xe9d/0x1240
> lock_acquire+0xd4/0x220
> __kernfs_remove+0x23c/0x2c0
> kernfs_remove+0x23/0x40
> sysfs_remove_dir+0x51/0x60
> kobject_del+0x18/0x50
> kobject_cleanup+0x4b/0x180
> kobject_put+0x2a/0x50
> __klp_free_patch+0x5b/0x60
> klp_free_patch_nowait+0x12/0x30
> klp_try_complete_transition+0x13e/0x180
> klp_transition_work_fn+0x26/0x40
> process_one_work+0x1d8/0x5d0
> worker_thread+0x4d/0x3d0
> kthread+0x113/0x150
> ret_from_fork+0x3a/0x50
>
> other info that might help us debug this:
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(klp_mutex);
> lock(kn->count#186);
> lock(klp_mutex);
> lock(kn->count#186);

Sigh, I overestimated the power of kobjects. I thought that this
must have been a false positive but it was not.

1. kernfs_fop_write() ignores kobj->kref. It takes care only
of its own reference count, see kernfs_get_active().

2. kobj_put() takes care only of kobj->kref. The following
code is called when the reference count reaches zero:

+ kobj_put()
+ kref_put()
+ kobject_release()
+ kobject_cleanup()
+ kobject_del()
+ sysfs_remove_dir()
+ kernfs_remove()
+ __kernfs_remove().
+ kernfs_drain()

, where kernfs_drain() waits until all opened files
are closed.

Now, we call kobj_put() under klp_mutex() when the sysfs
interface still exists. Files can be opened for
writing. As a result:

+ enabled_store() might wait for klp_mutex

+ kernfs_drain() would wait for enabled_store()
with klp_mutex() taken.


I have reproduced this with some extra sleeps.

I am going to work on another solution.

Best Regards,
Petr

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