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SubjectRe: [PATCH] mm: avoid slub allocation while holding list_lock
On Mon,  9 Sep 2019 00:10:16 -0600 Yu Zhao <yuzhao@google.com> 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.

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