Re: [PATCH] mm: slub: annotate kmem_cache_node->list_lock as raw_spinlock

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Apr 11, 2023 at 10:25:06PM +0800, Qi Zheng wrote:
> 
> 
> On 2023/4/11 22:19, Vlastimil Babka wrote:
> > On 4/11/23 16:08, Qi Zheng wrote:
> > > 
> > > 
> > > On 2023/4/11 21:40, Vlastimil Babka wrote:
> > > > On 4/11/23 15:08, Qi Zheng wrote:
> > > > > The list_lock can be held in the critical section of
> > > > > raw_spinlock, and then lockdep will complain about it
> > > > > like below:
> > > > > 
> > > > >    =============================
> > > > >    [ BUG: Invalid wait context ]
> > > > >    6.3.0-rc6-next-20230411 #7 Not tainted
> > > > >    -----------------------------
> > > > >    swapper/0/1 is trying to lock:
> > > > >    ffff888100055418 (&n->list_lock){....}-{3:3}, at: ___slab_alloc+0x73d/0x1330
> > > > >    other info that might help us debug this:
> > > > >    context-{5:5}
> > > > >    2 locks held by swapper/0/1:
> > > > >     #0: ffffffff824e8160 (rcu_tasks.cbs_gbl_lock){....}-{2:2}, at: cblist_init_generic+0x22/0x2d0
> > > > >     #1: ffff888136bede50 (&ACCESS_PRIVATE(rtpcp, lock)){....}-{2:2}, at: cblist_init_generic+0x232/0x2d0
> > > > >    stack backtrace:
> > > > >    CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.3.0-rc6-next-20230411 #7
> > > > >    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> > > > >    Call Trace:
> > > > >     <TASK>
> > > > >     dump_stack_lvl+0x77/0xc0
> > > > >     __lock_acquire+0xa65/0x2950
> > > > >     ? arch_stack_walk+0x65/0xf0
> > > > >     ? arch_stack_walk+0x65/0xf0
> > > > >     ? unwind_next_frame+0x602/0x8d0
> > > > >     lock_acquire+0xe0/0x300
> > > > >     ? ___slab_alloc+0x73d/0x1330
> > > > >     ? find_usage_forwards+0x39/0x50
> > > > >     ? check_irq_usage+0x162/0xa70
> > > > >     ? __bfs+0x10c/0x2c0
> > > > >     _raw_spin_lock_irqsave+0x4f/0x90
> > > > >     ? ___slab_alloc+0x73d/0x1330
> > > > >     ___slab_alloc+0x73d/0x1330
> > > > >     ? fill_pool+0x16b/0x2a0
> > > > >     ? look_up_lock_class+0x5d/0x160
> > > > >     ? register_lock_class+0x48/0x500
> > > > >     ? __lock_acquire+0xabc/0x2950
> > > > >     ? fill_pool+0x16b/0x2a0
> > > > >     kmem_cache_alloc+0x358/0x3b0
> > > > >     ? __lock_acquire+0xabc/0x2950
> > > > >     fill_pool+0x16b/0x2a0
> > > > >     ? __debug_object_init+0x292/0x560
> > > > >     ? lock_acquire+0xe0/0x300
> > > > >     ? cblist_init_generic+0x232/0x2d0
> > > > >     __debug_object_init+0x2c/0x560

This "__debug_object_init" is because INIT_WORK() is called in
cblist_init_generic(), so..

> > > > >     cblist_init_generic+0x147/0x2d0
> > > > >     rcu_init_tasks_generic+0x15/0x190
> > > > >     kernel_init_freeable+0x6e/0x3e0
> > > > >     ? rest_init+0x1e0/0x1e0
> > > > >     kernel_init+0x1b/0x1d0
> > > > >     ? rest_init+0x1e0/0x1e0
> > > > >     ret_from_fork+0x1f/0x30
> > > > >     </TASK>
> > > > > 
> > > > > The fill_pool() can only be called in the !PREEMPT_RT kernel
> > > > > or in the preemptible context of the PREEMPT_RT kernel, so
> > > > > the above warning is not a real issue, but it's better to
> > > > > annotate kmem_cache_node->list_lock as raw_spinlock to get
> > > > > rid of such issue.
> > > > 
> > > > + CC some RT and RCU people
> > > 
> > > Thanks.
> > > 
> > > > 
> > > > AFAIK raw_spinlock is not just an annotation, but on RT it changes the
> > > > implementation from preemptible mutex to actual spin lock, so it would be
> > > 
> > > Yeah.
> > > 
> > > > rather unfortunate to do that for a spurious warning. Can it be somehow
> > > > fixed in a better way?

... probably a better fix is to drop locks and call INIT_WORK(), or make
the cblist_init_generic() lockless (or part lockless), given it's just
initializing the cblist, it's probably doable. But I haven't taken a
careful look yet.

Regards,
Boqun

> > > 
> > > It's indeed unfortunate for the warning in the commit message. But
> > > functions like kmem_cache_alloc(GFP_ATOMIC) may indeed be called
> > > in the critical section of raw_spinlock or in the hardirq context, which
> > 
> > Hmm, I thought they may not, actually.
> > 
> > > will cause problem in the PREEMPT_RT kernel. So I still think it is
> > > reasonable to convert kmem_cache_node->list_lock to raw_spinlock type.
> > 
> > It wouldn't be the complete solution anyway. Once we allow even a GFP_ATOMIC
> > slab allocation for such context, it means also page allocation can happen
> > to refill the slabs, so lockdep will eventually complain about zone->lock,
> > and who knows what else.
> 
> Oh, indeed. :(
> 
> > 
> > > In addition, there are many fix patches for this kind of warning in the
> > > git log, so I also think there should be a general and better solution. :)
> > 
> > Maybe, but given above, I doubt it's this one.
> > 
> > > 
> > > > 
> > > 
> > 
> 
> -- 
> Thanks,
> Qi