Re: [PATCH v4] mm: Fix possible deadlock in kmemleak

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Aug 19, 2025 at 11:27:23PM -0400, Waiman Long wrote:
> On 8/18/25 5:09 AM, Gu Bowen wrote:
> > @@ -858,8 +870,14 @@ static void delete_object_part(unsigned long ptr, size_t size,
> >   	object = __find_and_remove_object(ptr, 1, objflags);
> >   	if (!object) {
> >   #ifdef DEBUG
> > +		/*
> > +		 * Printk deferring due to the kmemleak_lock held.
> > +		 * This is done to avoid deadlock.
> > +		 */
> > +		printk_deferred_enter();
> >   		kmemleak_warn("Partially freeing unknown object at 0x%08lx (size %zu)\n",
> >   			      ptr, size);
> > +		printk_deferred_exit();
> >   #endif
> 
> This particular warning message can be moved after unlock by adding a
> warning flag. Locking is done outside of the other two helper functions
> above, so it is easier to use printk_deferred_enter/exit() for those.

I thought about this as well but the above is under an #ifdef DEBUG so
we end up adding more lines on the unlock path (not sure which one looks
better; I'd say the above, marginally).

Another option would be to remove the #ifdef and try to identify the
call sites that trigger the warning. Last time I checked (many years
ago) they were fairly benign and decided to hide them before an #ifdef.

-- 
Catalin