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@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); 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
