Re: [PATCH v4 01/22] mm/slab: add rcu_barrier() to kvfree_rcu_barrier_on_cache()

Liam R. Howlett
2026-01-27T16:08:59Z

* Vlastimil Babka <vbabka@suse.cz> [260123 01:53]:
> After we submit the rcu_free sheaves to call_rcu() we need to make sure
> the rcu callbacks complete. kvfree_rcu_barrier() does that via
> flush_all_rcu_sheaves() but kvfree_rcu_barrier_on_cache() doesn't. Fix
> that.
>
> This currently causes no issues because the caches with sheaves we have
> are never destroyed. The problem flagged by kernel test robot was
> reported for a patch that enables sheaves for (almost) all caches, and
> occurred only with CONFIG_KASAN. Harry Yoo found the root cause [1]:
>
> It turns out the object freed by sheaf_flush_unused() was in KASAN
> percpu quarantine list (confirmed by dumping the list) by the time
> __kmem_cache_shutdown() returns an error.
>
> Quarantined objects are supposed to be flushed by kasan_cache_shutdown(),
> but things go wrong if the rcu callback (rcu_free_sheaf_nobarn()) is
> processed after kasan_cache_shutdown() finishes.
>
> That's why rcu_barrier() in __kmem_cache_shutdown() didn't help,
> because it's called after kasan_cache_shutdown().
>
> Calling rcu_barrier() in kvfree_rcu_barrier_on_cache() guarantees
> that it'll be added to the quarantine list before kasan_cache_shutdown()
> is called. So it's a valid fix!
>
> [1] https://lore.kernel.org/all/aWd6f3jERlrB5yeF@hyeyoo/
>
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202601121442.c530bed3-lkp@intel.com
> Fixes: 0f35040de593 ("mm/slab: introduce kvfree_rcu_barrier_on_cache() for cache destruction")
> Cc: stable@vger.kernel.org
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> Tested-by: Harry Yoo <harry.yoo@oracle.com>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
> mm/slab_common.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index eed7ea556cb1..ee994ec7f251 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -2133,8 +2133,11 @@ EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
> */
> void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
> {
> - if (s->cpu_sheaves)
> + if (s->cpu_sheaves) {
> flush_rcu_sheaves_on_cache(s);
> + rcu_barrier();
> + }
> +
> /*
> * TODO: Introduce a version of __kvfree_rcu_barrier() that works
> * on a specific slab cache.
>
> --
> 2.52.0
>
>