• Qian Cai's avatar
    mm/slub: fix a deadlock in show_slab_objects() · bb6932c5
    Qian Cai authored
    commit e4f8e513 upstream.
    
    A long time ago we fixed a similar deadlock in show_slab_objects() [1].
    However, it is apparently due to the commits like 01fb58bc ("slab:
    remove synchronous synchronize_sched() from memcg cache deactivation
    path") and 03afc0e2 ("slab: get_online_mems for
    kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
    just reading files in /sys/kernel/slab which will generate a lockdep
    splat below.
    
    Since the "mem_hotplug_lock" here is only to obtain a stable online node
    mask while racing with NUMA node hotplug, in the worst case, the results
    may me miscalculated while doing NUMA node hotplug, but they shall be
    corrected by later reads of the same files.
    
      WARNING: possible circular locking dependency detected
      ------------------------------------------------------
      cat/5224 is trying to acquire lock:
      ffff900012ac3120 (mem_hotplug_lock.rw_sem){++++}, at:
      show_slab_objects+0x94/0x3a8
    
      but task is already holding lock:
      b8ff009693eee398 (kn->count#45){++++}, at: kernfs_seq_start+0x44/0xf0
    
      which lock already depends on the new lock.
    
      the existing dependency chain (in reverse order) is:
    
      -> #2 (kn->count#45){++++}:
             lock_acquire+0x31c/0x360
             __kernfs_remove+0x290/0x490
             kernfs_remove+0x30/0x44
             sysfs_remove_dir+0x70/0x88
             kobject_del+0x50/0xb0
             sysfs_slab_unlink+0x2c/0x38
             shutdown_cache+0xa0/0xf0
             kmemcg_cache_shutdown_fn+0x1c/0x34
             kmemcg_workfn+0x44/0x64
             process_one_work+0x4f4/0x950
             worker_thread+0x390/0x4bc
             kthread+0x1cc/0x1e8
             ret_from_fork+0x10/0x18
    
      -> #1 (slab_mutex){+.+.}:
             lock_acquire+0x31c/0x360
             __mutex_lock_common+0x16c/0xf78
             mutex_lock_nested+0x40/0x50
             memcg_create_kmem_cache+0x38/0x16c
             memcg_kmem_cache_create_func+0x3c/0x70
             process_one_work+0x4f4/0x950
             worker_thread+0x390/0x4bc
             kthread+0x1cc/0x1e8
             ret_from_fork+0x10/0x18
    
      -> #0 (mem_hotplug_lock.rw_sem){++++}:
             validate_chain+0xd10/0x2bcc
             __lock_acquire+0x7f4/0xb8c
             lock_acquire+0x31c/0x360
             get_online_mems+0x54/0x150
             show_slab_objects+0x94/0x3a8
             total_objects_show+0x28/0x34
             slab_attr_show+0x38/0x54
             sysfs_kf_seq_show+0x198/0x2d4
             kernfs_seq_show+0xa4/0xcc
             seq_read+0x30c/0x8a8
             kernfs_fop_read+0xa8/0x314
             __vfs_read+0x88/0x20c
             vfs_read+0xd8/0x10c
             ksys_read+0xb0/0x120
             __arm64_sys_read+0x54/0x88
             el0_svc_handler+0x170/0x240
             el0_svc+0x8/0xc
    
      other info that might help us debug this:
    
      Chain exists of:
        mem_hotplug_lock.rw_sem --> slab_mutex --> kn->count#45
    
       Possible unsafe locking scenario:
    
             CPU0                    CPU1
             ----                    ----
        lock(kn->count#45);
                                     lock(slab_mutex);
                                     lock(kn->count#45);
        lock(mem_hotplug_lock.rw_sem);
    
       *** DEADLOCK ***
    
      3 locks held by cat/5224:
       #0: 9eff00095b14b2a0 (&p->lock){+.+.}, at: seq_read+0x4c/0x8a8
       #1: 0eff008997041480 (&of->mutex){+.+.}, at: kernfs_seq_start+0x34/0xf0
       #2: b8ff009693eee398 (kn->count#45){++++}, at:
      kernfs_seq_start+0x44/0xf0
    
      stack backtrace:
      Call trace:
       dump_backtrace+0x0/0x248
       show_stack+0x20/0x2c
       dump_stack+0xd0/0x140
       print_circular_bug+0x368/0x380
       check_noncircular+0x248/0x250
       validate_chain+0xd10/0x2bcc
       __lock_acquire+0x7f4/0xb8c
       lock_acquire+0x31c/0x360
       get_online_mems+0x54/0x150
       show_slab_objects+0x94/0x3a8
       total_objects_show+0x28/0x34
       slab_attr_show+0x38/0x54
       sysfs_kf_seq_show+0x198/0x2d4
       kernfs_seq_show+0xa4/0xcc
       seq_read+0x30c/0x8a8
       kernfs_fop_read+0xa8/0x314
       __vfs_read+0x88/0x20c
       vfs_read+0xd8/0x10c
       ksys_read+0xb0/0x120
       __arm64_sys_read+0x54/0x88
       el0_svc_handler+0x170/0x240
       el0_svc+0x8/0xc
    
    I think it is important to mention that this doesn't expose the
    show_slab_objects to use-after-free.  There is only a single path that
    might really race here and that is the slab hotplug notifier callback
    __kmem_cache_shrink (via slab_mem_going_offline_callback) but that path
    doesn't really destroy kmem_cache_node data structures.
    
    [1] http://lkml.iu.edu/hypermail/linux/kernel/1101.0/02850.html
    
    [akpm@linux-foundation.org: add comment explaining why we don't need mem_hotplug_lock]
    Link: http://lkml.kernel.org/r/1570192309-10132-1-git-send-email-cai@lca.pw
    Fixes: 01fb58bc ("slab: remove synchronous synchronize_sched() from memcg cache deactivation path")
    Fixes: 03afc0e2 ("slab: get_online_mems for kmem_cache_{create,destroy,shrink}")
    Signed-off-by: default avatarQian Cai <cai@lca.pw>
    Acked-by: default avatarMichal Hocko <mhocko@suse.com>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Pekka Enberg <penberg@kernel.org>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
    Cc: Roman Gushchin <guro@fb.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    bb6932c5
slub.c 142 KB