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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=fmwQKII2; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C7765606A32C; Wed, 4 Nov 2026 16:24:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=fmwQKII2 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 91E171F000C1; Wed, 4 Nov 2026 16:24:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793809492; bh=0jAbavI8ymZD98DRzmcyyK930fQyxtvawv4WSCcoeMY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=fmwQKII2OcOxpajVA5m6X2WFvwAjWfOocU2T+k3T70EY6soxn1uhskGNqKdz6OvIN VmsTpfWcJ5T2hMgM8ew3aaX5VkIgFgcNknEmUwKqhluTKM15Til1zGkd8qPNGzVRU4 9wtox6Lb4jWK0+GXsbHnqkx9ufit513GlaHrZx6yxW/3391rhDCQhMjsB7LHFTu/N1 Ea/N8tDlz9xWQxJc+6fVZrdyQ8SJqe2cpNdAzkBk+sSdys3aYDfbv9/nlAp3txGad0 grJf84a6ksmA9E335KvNDmCzIwyY7OWS2Hh52xgdE6jCnDQ4k4IX59FYf2NMjVZj3J aGcO6EHA1+JYA== Message-ID: <37ed51ad-5160-46b5-a951-a0da2ff1824d@kernel.org> Date: Wed, 4 Nov 2026 17:24:47 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH for-next-fixes v2 5/5] mm/slab: serialize deferred_work_barrier() Content-Language: en-US To: "Harry Yoo (Meta)" , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Alexei Starovoitov , Hao Li , Christoph Lameter , David Rientjes , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt Cc: kernel-team@meta.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Sashiko References: <20261103-kmalloc-nolock-fixes-v2-0-19167cfe4779@kernel.org> <20261103-kmalloc-nolock-fixes-v2-5-19167cfe4779@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/3/26 15:47, Harry Yoo (Meta) wrote: > irq_work_sync() uses rcuwait instead of busy waiting in two cases: > > 1. The kernel is using PREEMPT_RT and the irq work does not run in a > hardirq context. > > 2. The architecture cannot send inter-processor interrupts to make > busy waiting reasonably short. > > However, rcuwait.h says: >> The caller is responsible for locking around rcuwait_wait_event(), >> and [prepare_to/finish]_rcuwait() such that writes to @task are >> properly serialized. > > Since deferred_work_barrier() calls irq_work_sync() without any locks, > it can potentially cause a hang as writes to @task are not serialized. > > Fix this by serializing deferred_work_barrier() with a dedicated mutex. > We cannot reuse slab_mutex because e.g.) some users of > kvfree_rcu_barrier() are outside mm/. > > Reported-by: Sashiko > Closes: https://sashiko.dev/#/patchset/20260615-kfree_rcu_nolock-v3-0-70a54f3775bb%40kernel.org?part=5 > Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().") > Signed-off-by: Harry Yoo (Meta) Acked-by: Vlastimil Babka (SUSE) > --- > mm/slub.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/slub.c b/mm/slub.c > index 6dfa8ca3c937..50e88edcef3a 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -6636,8 +6636,12 @@ void defer_kfree_rcu(struct kvfree_rcu_head *head) > /* Must be called before flush_rcu_sheaves_on_cache() */ > void deferred_work_barrier(void) > { > + static DEFINE_MUTEX(barrier_mutex); > int cpu; > > + /* irq_work_sync() may use rcuwait that requires serialization */ > + guard(mutex)(&barrier_mutex); > + > for_each_possible_cpu(cpu) > irq_work_sync(&per_cpu_ptr(&deferred_percpu_work, cpu)->work); > } >