Alexander reported a circular lock dependency revealed by the mmap1 ltp
test:

  LOCKDEP_CIRCULAR (suite: ltp, case: mtest06 (mmap1))
          WARNING: possible circular locking dependency detected
          5.17.0-20220113.rc0.git0.f2211f194038.300.fc35.s390x+debug #1 Not tainted
          ------------------------------------------------------
          mmap1/202299 is trying to acquire lock:
          00000001892c0188 (css_set_lock){..-.}-{2:2}, at: obj_cgroup_release+0x4a/0xe0
          but task is already holding lock:
          00000000ca3b3818 (&sighand->siglock){-.-.}-{2:2}, at: force_sig_info_to_task+0x38/0x180
          which lock already depends on the new lock.
          the existing dependency chain (in reverse order) is:
          -> #1 (&sighand->siglock){-.-.}-{2:2}:
                 __lock_acquire+0x604/0xbd8
                 lock_acquire.part.0+0xe2/0x238
                 lock_acquire+0xb0/0x200
                 _raw_spin_lock_irqsave+0x6a/0xd8
                 __lock_task_sighand+0x90/0x190
                 cgroup_freeze_task+0x2e/0x90
                 cgroup_migrate_execute+0x11c/0x608
                 cgroup_update_dfl_csses+0x246/0x270
                 cgroup_subtree_control_write+0x238/0x518
                 kernfs_fop_write_iter+0x13e/0x1e0
                 new_sync_write+0x100/0x190
                 vfs_write+0x22c/0x2d8
                 ksys_write+0x6c/0xf8
                 __do_syscall+0x1da/0x208
                 system_call+0x82/0xb0
          -> #0 (css_set_lock){..-.}-{2:2}:
                 check_prev_add+0xe0/0xed8
                 validate_chain+0x736/0xb20
                 __lock_acquire+0x604/0xbd8
                 lock_acquire.part.0+0xe2/0x238
                 lock_acquire+0xb0/0x200
                 _raw_spin_lock_irqsave+0x6a/0xd8
                 obj_cgroup_release+0x4a/0xe0
                 percpu_ref_put_many.constprop.0+0x150/0x168
                 drain_obj_stock+0x94/0xe8
                 refill_obj_stock+0x94/0x278
                 obj_cgroup_charge+0x164/0x1d8
                 kmem_cache_alloc+0xac/0x528
                 __sigqueue_alloc+0x150/0x308
                 __send_signal+0x260/0x550
                 send_signal+0x7e/0x348
                 force_sig_info_to_task+0x104/0x180
                 force_sig_fault+0x48/0x58
                 __do_pgm_check+0x120/0x1f0
                 pgm_check_handler+0x11e/0x180
          other info that might help us debug this:
           Possible unsafe locking scenario:
                 CPU0                    CPU1
                 ----                    ----
            lock(&sighand->siglock);
                                         lock(css_set_lock);
                                         lock(&sighand->siglock);
            lock(css_set_lock);
           *** DEADLOCK ***
          2 locks held by mmap1/202299:
           #0: 00000000ca3b3818 (&sighand->siglock){-.-.}-{2:2}, at: force_sig_info_to_task+0x38/0x180
           #1: 00000001892ad560 (rcu_read_lock){....}-{1:2}, at: percpu_ref_put_many.constprop.0+0x0/0x168
          stack backtrace:
          CPU: 15 PID: 202299 Comm: mmap1 Not tainted 5.17.0-20220113.rc0.git0.f2211f194038.300.fc35.s390x+debug #1
          Hardware name: IBM 3906 M04 704 (LPAR)
          Call Trace:
            dump_stack_lvl+0x76/0x98
            check_noncircular+0x136/0x158
            check_prev_add+0xe0/0xed8
            validate_chain+0x736/0xb20
            __lock_acquire+0x604/0xbd8
            lock_acquire.part.0+0xe2/0x238
            lock_acquire+0xb0/0x200
            _raw_spin_lock_irqsave+0x6a/0xd8
            obj_cgroup_release+0x4a/0xe0
            percpu_ref_put_many.constprop.0+0x150/0x168
            drain_obj_stock+0x94/0xe8
            refill_obj_stock+0x94/0x278
            obj_cgroup_charge+0x164/0x1d8
            kmem_cache_alloc+0xac/0x528
            __sigqueue_alloc+0x150/0x308
            __send_signal+0x260/0x550
            send_signal+0x7e/0x348
            force_sig_info_to_task+0x104/0x180
            force_sig_fault+0x48/0x58
            __do_pgm_check+0x120/0x1f0
            pgm_check_handler+0x11e/0x180
          INFO: lockdep is turned off.

In this example a slab allocation from __send_signal() caused a
refilling and draining of a percpu objcg stock, resulted in a releasing
of another non-related objcg.  Objcg release path requires taking the
css_set_lock, which is used to synchronize objcg lists.

This can create a circular dependency with the sighandler lock, which is
taken with the locked css_set_lock by the freezer code (to freeze a
task).

In general it seems that using css_set_lock to synchronize objcg lists
makes any slab allocations and deallocation with the locked css_set_lock
and any intervened locks risky.

To fix the problem and make the code more robust let's stop using
css_set_lock to synchronize objcg lists and use a new dedicated spinlock
instead.

Link: https://lkml.kernel.org/r/Yfm1IHmoGdyUR81T@carbon.dhcp.thefacebook.com
Fixes: bf4f0599 ("mm: memcg/slab: obj_cgroup API")
Signed-off-by: Roman Gushchin <guro@fb.com>
Reported-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Tested-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>