Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback

From: "Jason A. Donenfeld" <[email-masked]>
Date: Thu, 13 Jun 2024 14:22:41 +0200

On Wed, Jun 12, 2024 at 08:38:02PM -0700, Paul E. McKenney wrote:
> o	Make the current kmem_cache_destroy() asynchronously wait for
> 	all memory to be returned, then complete the destruction.
> 	(This gets rid of a valuable debugging technique because
> 	in normal use, it is a bug to attempt to destroy a kmem_cache
> 	that has objects still allocated.)
> 
> o	Make a kmem_cache_destroy_rcu() that asynchronously waits for
> 	all memory to be returned, then completes the destruction.
> 	(This raises the question of what to is it takes a "long time"
> 	for the objects to be freed.)

These seem like the best two options.

> o	Make a kmem_cache_free_barrier() that blocks until all
> 	objects in the specified kmem_cache have been freed.
> 
> o	Make a kmem_cache_destroy_wait() that waits for all memory to
> 	be returned, then does the destruction.  This is equivalent to:
> 
> 		kmem_cache_free_barrier(&mycache);
> 		kmem_cache_destroy(&mycache);

These also seem fine, but I'm less keen about blocking behavior.

Though, along the ideas of kmem_cache_destroy_rcu(), you might also
consider renaming this last one to kmem_cache_destroy_rcu_wait/barrier().
This way, it's RCU focused, and you can deal directly with the question
of, "how long is too long to block/to memleak?"

Specifically what I mean is that we can still claim a memory leak has
occurred if one batched kfree_rcu freeing grace period has elapsed since
the last call to kmem_cache_destroy_rcu_wait/barrier() or
kmem_cache_destroy_rcu(). In that case, you quit blocking, or you quit
asynchronously waiting, and then you splat about a memleak like we have
now.

But then, if that mechanism generally works, we don't really need a new
function and we can just go with the first option of making
kmem_cache_destroy() asynchronously wait. It'll wait, as you described,
but then we adjust the tail of every kfree_rcu batch freeing cycle to
check if there are _still_ any old outstanding kmem_cache_destroy()
requests. If so, then we can splat and keep the old debugging info we
currently have for finding memleaks.

Jason