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@xxxxxxxx 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
