Re: [PATCH v4 29/35] mm: slub: Move flush_cpu_slab() invocations __free_slab() invocations out of IRQ context

[Vlastimil Babka]

On 8/9/2021 8:44 PM, Mike Galbraith wrote:
> On Mon, 2021-08-09 at 09:41 -0400, Qian Cai wrote:
>>
>>
>> On 8/5/2021 11:19 AM, Vlastimil Babka wrote:
>>>
>>>  
>>> +static DEFINE_MUTEX(flush_lock);
>>> +static DEFINE_PER_CPU(struct slub_flush_work, slub_flush);
>>> +
>>>  static void flush_all(struct kmem_cache *s)
>>>  {
>>> -       on_each_cpu_cond(has_cpu_slab, flush_cpu_slab, s, 1);
>>> +       struct slub_flush_work *sfw;
>>> +       unsigned int cpu;
>>> +
>>> +       mutex_lock(&flush_lock);
>>
>> Vlastimil, taking the lock here could trigger a warning during memory
>> offline/online due to the locking order:
>>
>> slab_mutex -> flush_lock
> 
> Bugger.  That chain ending with cpu_hotplug_lock makes slub_cpu_dead()
> taking slab_mutex a non-starter for cpu hotplug as well.  It's
> established early by kernel_init_freeable()..kmem_cache_destroy() as
> well as by slab_mem_going_offline_callback().

I suck at reading the lockdep splats, so I don't see yet how the "existing
reverse order" occurs - I do understand the order in the "lsbug".
What I also wonder is why didn't this occur also in the older RT trees with this
patch. I did change the order of locks in flush_all() to take flush_lock first
and cpus_read_lock() second, as Cyrill Gorcunov suggested. Would the original
order prevent this? Or we would fail anyway because we already took
cpus_read_lock() in offline_pages() and now are taking it again - do these nest
or not?