Re: [PATCH] mm/vmalloc: allow mem_dump_obj() to be called in interrupt context

[Andrew Morton]

On Sat, 12 Nov 2022 20:15:37 +0800 Zhen Lei <thunder.leizhen@xxxxxxxxxx> wrote:

> The function mem_dump_obj() can sometimes provide valuable debugging
> information, but it cannot be called in an interrupt context because
> spinlock vmap_area_lock has not been protected against IRQs. If the
> current task has held the lock before hard/soft interrupt handler calls
> mem_dump_obj(), simply abandoning the dump operation can avoid deadlock.
> That is, no deadlock occurs in extreme cases, and dump succeeds in most
> cases.
> 
> ...
>
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4034,6 +4034,9 @@ bool vmalloc_dump_obj(void *object)
>  	struct vm_struct *vm;
>  	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
>  
> +	if (unlikely(spin_is_locked(&vmap_area_lock)))
> +		return false;
> +
>  	vm = find_vm_area(objp);
>  	if (!vm)
>  		return false;

Yes, but this will worsen the current uses of this function.  Consider
the case where task A wants to call vmalloc_dump_obj() but task B holds
vmap_area_lock.  No problem, task A will simply spin until task B is
done.

But after this patch, task A's call to vmalloc_dump_obj() will return
without having done anything.