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Damien Le Moal reports a lockdep splat with the acpi_power_meter, observed with Linux v5.5 and later. ====================================================== WARNING: possible circular locking dependency detected 5.6.0-rc2+ #629 Not tainted ------------------------------------------------------ python/1397 is trying to acquire lock: ffff888619080070 (&resource->lock){+.+.}, at: show_power+0x3c/0xa0 [acpi_power_meter] but task is already holding lock: ffff88881643f188 (kn->count#119){++++}, at: kernfs_seq_start+0x6a/0x160 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #1 (kn->count#119){++++}: __kernfs_remove+0x626/0x7e0 kernfs_remove_by_name_ns+0x41/0x80 remove_attrs+0xcb/0x3c0 [acpi_power_meter] acpi_power_meter_notify+0x1f7/0x310 [acpi_power_meter] acpi_ev_notify_dispatch+0x198/0x1f3 acpi_os_execute_deferred+0x4d/0x70 process_one_work+0x7c8/0x1340 worker_thread+0x94/0xc70 kthread+0x2ed/0x3f0 ret_from_fork+0x24/0x30 -> #0 (&resource->lock){+.+.}: __lock_acquire+0x20be/0x49b0 lock_acquire+0x127/0x340 __mutex_lock+0x15b/0x1350 show_power+0x3c/0xa0 [acpi_power_meter] dev_attr_show+0x3f/0x80 sysfs_kf_seq_show+0x216/0x410 seq_read+0x407/0xf90 vfs_read+0x152/0x2c0 ksys_read+0xf3/0x1d0 do_syscall_64+0x95/0x1010 entry_SYSCALL_64_after_hwframe+0x49/0xbe other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(kn->count#119); lock(&resource->lock); lock(kn->count#119); lock(&resource->lock); *** DEADLOCK *** 4 locks held by python/1397: #0: ffff8890242d64e0 (&f->f_pos_lock){+.+.}, at: __fdget_pos+0x9b/0xb0 #1: ffff889040be74e0 (&p->lock){+.+.}, at: seq_read+0x6b/0xf90 #2: ffff8890448eb880 (&of->mutex){+.+.}, at: kernfs_seq_start+0x47/0x160 #3: ffff88881643f188 (kn->count#119){++++}, at: kernfs_seq_start+0x6a/0x160 stack backtrace: CPU: 10 PID: 1397 Comm: python Not tainted 5.6.0-rc2+ #629 Hardware name: Supermicro Super Server/X11DPL-i, BIOS 3.1 05/21/2019 Call Trace: dump_stack+0x97/0xe0 check_noncircular+0x32e/0x3e0 ? print_circular_bug.isra.0+0x1e0/0x1e0 ? unwind_next_frame+0xb9a/0x1890 ? entry_SYSCALL_64_after_hwframe+0x49/0xbe ? graph_lock+0x79/0x170 ? __lockdep_reset_lock+0x3c0/0x3c0 ? mark_lock+0xbc/0x1150 __lock_acquire+0x20be/0x49b0 ? mark_held_locks+0xe0/0xe0 ? stack_trace_save+0x91/0xc0 lock_acquire+0x127/0x340 ? show_power+0x3c/0xa0 [acpi_power_meter] ? device_remove_bin_file+0x10/0x10 ? device_remove_bin_file+0x10/0x10 __mutex_lock+0x15b/0x1350 ? show_power+0x3c/0xa0 [acpi_power_meter] ? show_power+0x3c/0xa0 [acpi_power_meter] ? mutex_lock_io_nested+0x11f0/0x11f0 ? lock_downgrade+0x6a0/0x6a0 ? kernfs_seq_start+0x47/0x160 ? lock_acquire+0x127/0x340 ? kernfs_seq_start+0x6a/0x160 ? device_remove_bin_file+0x10/0x10 ? show_power+0x3c/0xa0 [acpi_power_meter] show_power+0x3c/0xa0 [acpi_power_meter] dev_attr_show+0x3f/0x80 ? memset+0x20/0x40 sysfs_kf_seq_show+0x216/0x410 seq_read+0x407/0xf90 ? security_file_permission+0x16f/0x2c0 vfs_read+0x152/0x2c0 Problem is that reading an attribute takes the kernfs lock in the kernfs code, then resource->lock in the driver. During an ACPI notification, the opposite happens: The resource lock is taken first, followed by the kernfs lock when sysfs attributes are removed and re-created. Presumably this is now seen due to some locking related changes in kernfs after v5.4, but it was likely always a problem. Fix the problem by not blindly acquiring the lock in the notification function. It is only needed to protect the various update functions. However, those update functions are called anyway when sysfs attributes are read. This means that we can just stop calling those functions from the notifier, and the resource lock in the notifier function is no longer needed. That leaves two situations: First, METER_NOTIFY_CONFIG removes and re-allocates capability strings. While it did so under the resource lock, _displaying_ those strings was not protected, creating a race condition. To solve this problem, selectively protect both removal/creation and reporting of capability attributes with the resource lock. Second, removing and re-creating the attribute files is no longer protected by the resource lock. That doesn't matter since access to each individual attribute is protected by the kernfs lock. Userspace may get messed up if attributes disappear and reappear under its nose, but that is not different than today, and there is nothing we can do about it without major driver restructuring. Last but not least, when removing the driver, remove attribute functions first, then release capability strings. This avoids yet another race condition. Reported-by: Damien Le Moal <Damien.LeMoal@wdc.com> Cc: Damien Le Moal <Damien.LeMoal@wdc.com> Cc: stable@vger.kernel.org # v5.5+ Tested-by: Damien Le Moal <damien.lemoal@wdc.com> Signed-off-by: Guenter Roeck <linux@roeck-us.net> |
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