Re: [PATCH] mm: avoid slub allocation while holding list_lock

[Andrew Morton]

On Mon,  9 Sep 2019 00:10:16 -0600 Yu Zhao <yuzhao@xxxxxxxxxx> wrote:

> If we are already under list_lock, don't call kmalloc(). Otherwise we
> will run into deadlock because kmalloc() also tries to grab the same
> lock.
> 
> Instead, allocate pages directly. Given currently page->objects has
> 15 bits, we only need 1 page. We may waste some memory but we only do
> so when slub debug is on.
> 
>   WARNING: possible recursive locking detected
>   --------------------------------------------
>   mount-encrypted/4921 is trying to acquire lock:
>   (&(&n->list_lock)->rlock){-.-.}, at: ___slab_alloc+0x104/0x437
> 
>   but task is already holding lock:
>   (&(&n->list_lock)->rlock){-.-.}, at: __kmem_cache_shutdown+0x81/0x3cb
> 
>   other info that might help us debug this:
>    Possible unsafe locking scenario:
> 
>          CPU0
>          ----
>     lock(&(&n->list_lock)->rlock);
>     lock(&(&n->list_lock)->rlock);
> 
>    *** DEADLOCK ***
> 

It would be better if a silly low-level debug function like this
weren't to try to allocate memory at all.  Did you consider simply
using a statically allocated buffer?

{
	static char buffer[something large enough];
	static spinlock_t lock_to_protect_it;


Alternatively, do we need to call get_map() at all in there?  We could
simply open-code the get_map() functionality inside
list_slab_objects().  It would be slower, but printk is already slow. 
Potentially extremely slow.