Re: [PATCH v5.6-rt] mm: slub: Always flush the delayed empty slubs in flush_all()

[David Rientjes <rientjes@google.com>]

On Mon, 4 May 2020, Kevin Hao wrote:

> After commit f0b231101c94 ("mm/SLUB: delay giving back empty slubs to
> IRQ enabled regions"), when the free_slab() is invoked with the IRQ
> disabled, the empty slubs are moved to a per-CPU list and will be
> freed after IRQ enabled later. But in the current codes, there is
> a check to see if there really has the cpu slub on a specific cpu
> before flushing the delayed empty slubs, this may cause a reference
> of already released kmem_cache in a scenario like below:
> 	cpu 0				cpu 1
>   kmem_cache_destroy()
>     flush_all()
>                          --->IPI       flush_cpu_slab()
>                                          flush_slab()
>                                            deactivate_slab()
>                                              discard_slab()
>                                                free_slab()
>                                              c->page = NULL;
>       for_each_online_cpu(cpu)
>         if (!has_cpu_slab(1, s))
>           continue
>         this skip to flush the delayed
>         empty slub released by cpu1
>     kmem_cache_free(kmem_cache, s)
> 
>                                        kmalloc()
>                                          __slab_alloc()
>                                             free_delayed()
>                                             __free_slab()
>                                             reference to released kmem_cache
> 
> Fixes: f0b231101c94 ("mm/SLUB: delay giving back empty slubs to IRQ enabled regions")
> Signed-off-by: Kevin Hao <haokexin@gmail.com>

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