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

Jason A. Donenfeld
2024-06-13T00:32:02Z

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/20240612133357.2596-1-linus.luessing@c0d3.blue/
> > > 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