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 02:31:53 +0200

On Thu, Jun 13, 2024 at 01:31:57AM +0200, Jason A. Donenfeld wrote:
> On Wed, Jun 12, 2024 at 03:37:55PM -0700, Paul E. McKenney wrote:
> > On Wed, Jun 12, 2024 at 02:33:05PM -0700, Jakub Kicinski wrote:
> > > On Sun,  9 Jun 2024 10:27:12 +0200 Julia Lawall wrote:
> > > > Since SLOB was removed, it is not necessary to use call_rcu
> > > > when the callback only performs kmem_cache_free. Use
> > > > kfree_rcu() directly.
> > > > 
> > > > The changes were done using the following Coccinelle semantic patch.
> > > > This semantic patch is designed to ignore cases where the callback
> > > > function is used in another way.
> > > 
> > > How does the discussion on:
> > >   [PATCH] Revert "batman-adv: prefer kfree_rcu() over call_rcu() with free-only callbacks"
> > >   https://lore.kernel.org/all/[email-masked]/
> > > reflect on this series? IIUC we should hold off..
> > 
> > We do need to hold off for the ones in kernel modules (such as 07/14)
> > where the kmem_cache is destroyed during module unload.
> > 
> > OK, I might as well go through them...
> > 
> > [PATCH 01/14] wireguard: allowedips: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
> > 	Needs to wait, see wg_allowedips_slab_uninit().
> 
> Right, this has exactly the same pattern as the batman-adv issue:
> 
>     void wg_allowedips_slab_uninit(void)
>     {
>             rcu_barrier();
>             kmem_cache_destroy(node_cache);
>     }
> 
> I'll hold off on sending that up until this matter is resolved.

BTW, I think this whole thing might be caused by:

    a35d16905efc ("rcu: Add basic support for kfree_rcu() batching")

The commit message there mentions:

    There is an implication with rcu_barrier() with this patch. Since the
    kfree_rcu() calls can be batched, and may not be handed yet to the RCU
    machinery in fact, the monitor may not have even run yet to do the
    queue_rcu_work(), there seems no easy way of implementing rcu_barrier()
    to wait for those kfree_rcu()s that are already made. So this means a
    kfree_rcu() followed by an rcu_barrier() does not imply that memory will
    be freed once rcu_barrier() returns.

Before that, a kfree_rcu() used to just add a normal call_rcu() into the
list, but with the function offset < 4096 as a special marker. So the
kfree_rcu() calls would be treated alongside the other call_rcu() ones
and thus affected by rcu_barrier(). Looks like that behavior is no more
since this commit.

Rather than getting rid of the batching, which seems good for
efficiency, I wonder if the right fix to this would be adding a
`should_destroy` boolean to kmem_cache, which kmem_cache_destroy() sets
to true. And then right after it checks `if (number_of_allocations == 0)
actually_destroy()`, and likewise on each kmem_cache_free(), it could
check `if (should_destroy && number_of_allocations == 0)
actually_destroy()`. This way, the work is delayed until it's safe to do
so. This might also mitigate other lurking bugs of bad code that calls
kmem_cache_destroy() before kmem_cache_free().

Jason