Re: Fwd: Debugger for mapcount/reference counting issues

[Omar Sandoval <osandov@osandov.com>]

On Fri, Mar 28, 2025 at 08:24:47PM -0700, Adrian Johnston wrote:
> I was just reading an article about the long running
> mapcount/reference counting issues in the kernel over on LWN.
> https://lwn.net/Articles/1013956/bigpage
> 
> And I'd like to make a recommendation in this ongoing discussion.
> 
> The kernel would benefit from a reference count debugger.  This is a
> debug build only tool that keeps track of the owner of each increment
> to a reference count and then confirms that each call to release a
> reference count has an owner that matches an increment by a previously
> registered owner.  This enforces an ownership match between allocation
> and release of each reference count increment, even in ad hoc code. It
> would also be able to validate the reference counter independently
> when resources are being processed, created and destroyed.
> 
> Ideally this tool would be able to show calls stacks associated with
> release operations that have unregistered owners and call stacks for
> leaked reference increments at system shutdown.  Having this type of
> tool enabled on your integration build (linux-next) might save you a
> lot of debugging.
> 
> Personally, I don't think this is a good problem to have and would
> argue that every resource should have a single owner that arbitrates
> it's life cycle.  But there are tools that can help if reference
> counting is necessary.
> 
> All the best,
> Adrian Johnston

Hi,

What you described sounds very similar to ref_tracker:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/ref_tracker.h
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4e66934eaadc83b27ada8d42b60894018f3bfabf

It doesn't seem to be used very widely, but it has proven useful for
debugging network namespace leaks.

Thanks,
Omar