Deadlock between CPU offline and kmem_cache_create

["Paul E. McKenney" <paulmck@kernel.org>]

Hello!

Commit 1f0723a4c0df ("mm, slub: enable slub_debug static key when creating
cache with explicit debug flags") results in the lockdep complaint (see
below) in the presence of CPU-hotplug offline operations.  I triggered
and bisected this using the following command:

tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 10 --configs TREE05 --trust-make

This triggers when rcutorture's kmem_cache_create() for its testing of
mem_dump_obj().  When I reverted commit 1f0723a4c0df, the splat went away.

I tried moving rcutorture's mem_dump_obj() testing to rcutorture's
module-init function, but that did not help.  In retrospect, this is no
surprise because the deadlock is caused by the call to kmem_cache_create()
and the slab CPU-hotplug notifiers.  There is no lock in this deadlock
cycle that is under rcutorture's control.

I could imagine moving the static_branch_enable() out into a clean
environment, but this would of course require some mechanism to make
sure that the slab was still in existence at that time.  One way to do
this would be to have a timer that is set at the site of the current
static_branch_enable() and deleted at slab-deletion time.  Plus there
would be a short period of time when debugging would not yet be enabled
for this slab (would that be a problem?).

This time could be minimized using (say) an hrtimer timeout of 1
microsecond or some such.  It could be eliminated by having the timer
handler do a wakeup that kmem_cache_create() waits for at some point
after it releases slab_mutex.

Alternatively, maybe some way can be found to avoid acquiring slab_mutex
in slab's CPU-hotplug notifiers.

Thoughts?

							Thanx, Paul

------------------------------------------------------------------------

[  602.429399] ======================================================
[  602.429777] WARNING: possible circular locking dependency detected
[  602.430156] 5.12.0+ #15 Not tainted
[  602.430374] ------------------------------------------------------
[  602.430759] rcu_torture_sta/109 is trying to acquire lock:
[  602.431099] ffffffff96063cd0 (cpu_hotplug_lock){++++}-{0:0}, at: static_key_enable+0x9/0x20
[  602.431630] 
[  602.431630] but task is already holding lock:
[  602.431992] ffffffff96173c28 (slab_mutex){+.+.}-{3:3}, at: kmem_cache_create_usercopy+0x2d/0x250
[  602.432541] 
[  602.432541] which lock already depends on the new lock.
[  602.432541] 
[  602.433039] 
[  602.433039] the existing dependency chain (in reverse order) is:
[  602.433494] 
[  602.433494] -> #1 (slab_mutex){+.+.}-{3:3}:
[  602.433842]        lock_acquire+0xb9/0x3a0
[  602.434107]        __mutex_lock+0x8d/0x920
[  602.434366]        slub_cpu_dead+0x15/0xf0
[  602.434625]        cpuhp_invoke_callback+0x17a/0x7c0
[  602.434938]        cpuhp_invoke_callback_range+0x3b/0x80
[  602.435266]        _cpu_down+0xdf/0x2a0
[  602.435504]        cpu_down+0x2c/0x50
[  602.435734]        device_offline+0x82/0xb0
[  602.436005]        remove_cpu+0x1a/0x30
[  602.436243]        torture_offline+0x80/0x140
[  602.436514]        torture_onoff+0x147/0x260
[  602.436778]        kthread+0x10a/0x140
[  602.437013]        ret_from_fork+0x22/0x30
[  602.437274] 
[  602.437274] -> #0 (cpu_hotplug_lock){++++}-{0:0}:
[  602.437654]        check_prev_add+0x8f/0xbf0
[  602.437919]        __lock_acquire+0x13f0/0x1d80
[  602.438198]        lock_acquire+0xb9/0x3a0
[  602.438452]        cpus_read_lock+0x21/0xa0
[  602.438713]        static_key_enable+0x9/0x20
[  602.438985]        __kmem_cache_create+0x38d/0x430
[  602.439284]        kmem_cache_create_usercopy+0x146/0x250
[  602.439619]        kmem_cache_create+0xd/0x10
[  602.439895]        rcu_torture_stats+0x79/0x280
[  602.440179]        kthread+0x10a/0x140
[  602.440413]        ret_from_fork+0x22/0x30
[  602.440669] 
[  602.440669] other info that might help us debug this:
[  602.440669] 
[  602.441154]  Possible unsafe locking scenario:
[  602.441154] 
[  602.441523]        CPU0                    CPU1
[  602.441803]        ----                    ----
[  602.442085]   lock(slab_mutex);
[  602.442281]                                lock(cpu_hotplug_lock);
[  602.442662]                                lock(slab_mutex);
[  602.443009]   lock(cpu_hotplug_lock);
[  602.443239] 
[  602.443239]  *** DEADLOCK ***
[  602.443239] 
[  602.443606] 1 lock held by rcu_torture_sta/109:
[  602.443892]  #0: ffffffff96173c28 (slab_mutex){+.+.}-{3:3}, at: kmem_cache_create_usercopy+0x2d/0x250
[  602.444472] 
[  602.444472] stack backtrace:
[  602.444743] CPU: 3 PID: 109 Comm: rcu_torture_sta Not tainted 5.12.0+ #15
[  602.445176] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014
[  602.445728] Call Trace:
[  602.445891]  dump_stack+0x6d/0x89
[  602.446116]  check_noncircular+0xfe/0x110
[  602.446401]  ? lock_is_held_type+0x98/0x110
[  602.446664]  check_prev_add+0x8f/0xbf0
[  602.446908]  __lock_acquire+0x13f0/0x1d80
[  602.447162]  lock_acquire+0xb9/0x3a0
[  602.447385]  ? static_key_enable+0x9/0x20
[  602.447640]  ? mark_held_locks+0x49/0x70
[  602.447894]  cpus_read_lock+0x21/0xa0
[  602.448124]  ? static_key_enable+0x9/0x20
[  602.448373]  static_key_enable+0x9/0x20
[  602.448614]  __kmem_cache_create+0x38d/0x430
[  602.448882]  kmem_cache_create_usercopy+0x146/0x250
[  602.449184]  ? rcu_torture_stats_print+0xd0/0xd0
[  602.449469]  kmem_cache_create+0xd/0x10
[  602.449708]  rcu_torture_stats+0x79/0x280
[  602.449964]  ? rcu_torture_stats_print+0xd0/0xd0
[  602.450251]  kthread+0x10a/0x140
[  602.450452]  ? kthread_park+0x80/0x80
[  602.450682]  ret_from_fork+0x22/0x30