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SubjectRe: [PATCH v12 00/12]
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);

*** DEADLOCK ***

3 locks held by kworker/1:1/49: #0: 00000000654f4e5a ((wq_completion)"events"){+.+.}, at:
process_one_work+0x153/0x5d0 #1: 000000003c1dc846 ((klp_transition_work).work){+.+.}, at:
process_one_work+0x153/0x5d0 #2: 0000000073632424 (klp_mutex){+.+.}, at: klp_transition_work_fn+0x17/0x40

stack backtrace:
CPU: 1 PID: 49 Comm: kworker/1:1 Tainted: G K
4.17.0-rc1-klp_replace_v12-117114-gfedb3eba611d #218
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.0.0-prebuilt.qemu-project.org 04/01/2014
Workqueue: events klp_transition_work_fn
Call Trace:
dump_stack+0x81/0xb8
print_circular_bug.isra.39+0x200/0x20e
check_prev_add.constprop.47+0x725/0x740
? print_shortest_lock_dependencies+0x1c0/0x1c0
__lock_acquire+0xe9d/0x1240
lock_acquire+0xd4/0x220
? kernfs_remove+0x23/0x40
__kernfs_remove+0x23c/0x2c0
? kernfs_remove+0x23/0x40
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
? process_one_work+0x153/0x5d0
worker_thread+0x4d/0x3d0
? trace_hardirqs_on+0xd/0x10
kthread+0x113/0x150
? process_one_work+0x5d0/0x5d0
? kthread_delayed_work_timer_fn+0x90/0x90
? kthread_delayed_work_timer_fn+0x90/0x90
ret_from_fork+0x3a/0x50


I think it could be related to registration removal and API changes. One
thread writes to sysfs and wants to take klp_mutex there (CPU#1), the
other holds klp_mutex in a transition period and calls klp_free_patch()
to remove the sysfs infrastructure.

Regards,
Miroslav

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