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

[Qi Zheng]

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
> >    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?

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
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.

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. :)

>

--
Thanks,
Qi