Re: [PATCH 5.13] mm, slub: move slub_debug static key enabling outside slab_mutex

[David Rientjes <rientjes@google.com>]

On Tue, 4 May 2021, Vlastimil Babka wrote:

> On 5/4/21 2:00 PM, Vlastimil Babka wrote:
> > Paul E. McKenney reported [1] that commit 1f0723a4c0df ("mm, slub: enable
> > slub_debug static key when creating cache with explicit debug flags") results
> > in the lockdep complaint:
> > 
> >  ======================================================
> >  WARNING: possible circular locking dependency detected
> >  5.12.0+ #15 Not tainted
> >  ------------------------------------------------------
> >  rcu_torture_sta/109 is trying to acquire lock:
> >  ffffffff96063cd0 (cpu_hotplug_lock){++++}-{0:0}, at: static_key_enable+0x9/0x20
> > 
> >  but task is already holding lock:
> >  ffffffff96173c28 (slab_mutex){+.+.}-{3:3}, at: kmem_cache_create_usercopy+0x2d/0x250
> > 
> >  which lock already depends on the new lock.
> > 
> >  the existing dependency chain (in reverse order) is:
> > 
> >  -> #1 (slab_mutex){+.+.}-{3:3}:
> >         lock_acquire+0xb9/0x3a0
> >         __mutex_lock+0x8d/0x920
> >         slub_cpu_dead+0x15/0xf0
> >         cpuhp_invoke_callback+0x17a/0x7c0
> >         cpuhp_invoke_callback_range+0x3b/0x80
> >         _cpu_down+0xdf/0x2a0
> >         cpu_down+0x2c/0x50
> >         device_offline+0x82/0xb0
> >         remove_cpu+0x1a/0x30
> >         torture_offline+0x80/0x140
> >         torture_onoff+0x147/0x260
> >         kthread+0x10a/0x140
> >         ret_from_fork+0x22/0x30
> > 
> >  -> #0 (cpu_hotplug_lock){++++}-{0:0}:
> >         check_prev_add+0x8f/0xbf0
> >         __lock_acquire+0x13f0/0x1d80
> >         lock_acquire+0xb9/0x3a0
> >         cpus_read_lock+0x21/0xa0
> >         static_key_enable+0x9/0x20
> >         __kmem_cache_create+0x38d/0x430
> >         kmem_cache_create_usercopy+0x146/0x250
> >         kmem_cache_create+0xd/0x10
> >         rcu_torture_stats+0x79/0x280
> >         kthread+0x10a/0x140
> >         ret_from_fork+0x22/0x30
> > 
> >  other info that might help us debug this:
> > 
> >   Possible unsafe locking scenario:
> > 
> >         CPU0                    CPU1
> >         ----                    ----
> >    lock(slab_mutex);
> >                                 lock(cpu_hotplug_lock);
> >                                 lock(slab_mutex);
> >    lock(cpu_hotplug_lock);
> > 
> >   *** DEADLOCK ***
> > 
> >  1 lock held by rcu_torture_sta/109:
> >   #0: ffffffff96173c28 (slab_mutex){+.+.}-{3:3}, at: kmem_cache_create_usercopy+0x2d/0x250
> > 
> >  stack backtrace:
> >  CPU: 3 PID: 109 Comm: rcu_torture_sta Not tainted 5.12.0+ #15
> >  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014
> >  Call Trace:
> >   dump_stack+0x6d/0x89
> >   check_noncircular+0xfe/0x110
> >   ? lock_is_held_type+0x98/0x110
> >   check_prev_add+0x8f/0xbf0
> >   __lock_acquire+0x13f0/0x1d80
> >   lock_acquire+0xb9/0x3a0
> >   ? static_key_enable+0x9/0x20
> >   ? mark_held_locks+0x49/0x70
> >   cpus_read_lock+0x21/0xa0
> >   ? static_key_enable+0x9/0x20
> >   static_key_enable+0x9/0x20
> >   __kmem_cache_create+0x38d/0x430
> >   kmem_cache_create_usercopy+0x146/0x250
> >   ? rcu_torture_stats_print+0xd0/0xd0
> >   kmem_cache_create+0xd/0x10
> >   rcu_torture_stats+0x79/0x280
> >   ? rcu_torture_stats_print+0xd0/0xd0
> >   kthread+0x10a/0x140
> >   ? kthread_park+0x80/0x80
> >   ret_from_fork+0x22/0x30
> > 
> > This is because there's one order of locking from the hotplug callbacks:
> > 
> > lock(cpu_hotplug_lock); // from hotplug machinery itself
> > lock(slab_mutex); // in e.g. slab_mem_going_offline_callback()
> > 
> > And commit 1f0723a4c0df made the reverse sequence possible:
> > lock(slab_mutex); // in kmem_cache_create_usercopy()
> > lock(cpu_hotplug_lock); // kmem_cache_open() -> static_key_enable()
> > 
> > The simplest fix is to move static_key_enable() to a place before slab_mutex is
> > taken. That means kmem_cache_create_usercopy() in mm/slab_common.c which is not
> > ideal for SLUB-specific code, but the #ifdef CONFIG_SLUB_DEBUG makes it
> > at least self-contained and obvious.
> > 
> > [1] https://lore.kernel.org/lkml/20210502171827.GA3670492@paulmck-ThinkPad-P17-Gen-1/
> > 
> > Reported-and-tested-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Forgot:
> Fixes: 1f0723a4c0df ("mm, slub: enable slub_debug static key when creating cache
> with explicit debug flags")
> 

Acked-by: David Rientjes <rientjes@google.com>

Nice concise fix!