linux-kernel - Re: [PATCH v4 01/22] mm/slab: add rcu_barrier() to kvfree_rcu_barrier_on

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Message-ID: <cgkr4xc5oczrjiox2utksbvecbke2kpniacaog36njcdmvkdxx@6hnvksdzrwja>
Date: Tue, 27 Jan 2026 11:08:07 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Harry Yoo <harry.yoo@...cle.com>, Petr Tesarik <ptesarik@...e.com>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>, Hao Li <hao.li@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
        bpf@...r.kernel.org, kasan-dev@...glegroups.com,
        kernel test robot <oliver.sang@...el.com>, stable@...r.kernel.org
Subject: Re: [PATCH v4 01/22] mm/slab: add rcu_barrier() to
 kvfree_rcu_barrier_on_cache()

* Vlastimil Babka <vbabka@...e.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@...el.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@...r.kernel.org
> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> Tested-by: Harry Yoo <harry.yoo@...cle.com>
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.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
> 
>