Re: [PATCH] mm:Avoid soft lockup due to possible attempt of double locking object's lock in __delete_object

Catalin Marinas
2016-08-31T07:54:27Z

On Tue, Aug 30, 2016 at 02:35:12PM -0400, Nicholas Krause wrote:
> This fixes a issue in the current locking logic of the function,
> __delete_object where we are trying to attempt to lock the passed
> object structure's spinlock again after being previously held
> elsewhere by the kmemleak code. Fix this by instead of assuming
> we are the only one contending for the object's lock their are
> possible other users and create two branches, one where we get
> the lock when calling spin_trylock_irqsave on the object's lock
> and the other when the lock is held else where by kmemleak.

Have you actually got a deadlock that requires this fix?

> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -631,12 +631,19 @@ static void __delete_object(struct kmemleak_object *object)
>
> /*
> * Locking here also ensures that the corresponding memory block
> - * cannot be freed when it is being scanned.
> + * cannot be freed when it is being scanned. Further more the
> + * object's lock may have been previously holded by another holder
> + * in the kmemleak code, therefore attempt to lock the object's lock
> + * before holding it and unlocking it.
> */
> - spin_lock_irqsave(&object->lock, flags);
> - object->flags &= ~OBJECT_ALLOCATED;
> - spin_unlock_irqrestore(&object->lock, flags);
> - put_object(object);
> + if (spin_trylock_irqsave(&object->lock, flags)) {
> + object->flags &= ~OBJECT_ALLOCATED;
> + spin_unlock_irqrestore(&object->lock, flags);
> + put_object(object);
> + } else {
> + object->flags &= ~OBJECT_ALLOCATED;
> + put_object(object);
> + }

NAK. This lock here is needed, as described in the comment, to prevent
an object being freed while it is being scanned. The scan_object()
function acquires the same lock and checks for OBJECT_ALLOCATED before
accessing the memory (which could be vmalloc'ed for example, so freeing
would cause a page fault).

--
Catalin