From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2CAA141A76D; Mon, 20 Jul 2026 12:45:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784551559; cv=none; b=L9DogLHOKrBl/TljbrAYuU3GlV1bYAIy7N8lUAjLFddbz6wumzUjkbChy9nXfDBUJ6NitC+9/eBfxstP3TyaRJDkTE84ioKci1ZKU4ymKcLgZyh/mgyhGmmiWwdp6JcpvRFk6KbN/aa0J1JNDEU8/NQ8V5iE4FkVk+UxbTJX4rM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784551559; c=relaxed/simple; bh=cFwkADh+XAG2oI8Bdc9dWbLEIl5vVV9vjBqbY6QkSLE=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=a0eZmDLAdBQEHy0LXkSqMmUAJ+6n9aBz2hOsfdLsakgnjQq9cCn4Qv2yj2IMsPIpVx1sXV8bhaj5jEuV4GB1skzk3E7F1Pdx/CGBmQRnWS36wb7/Zr3i94XyEq9/3q76caui5nNzKz6Me2ROllOryqIObReErlO2x+9eU7NHNCY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=j93iNg0a; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="j93iNg0a" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 472DE1F00A3D; Mon, 20 Jul 2026 12:45:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784551558; bh=bu4nv7yTgUcNSgTheBamJjAfAc2vPVO37Yi1zmMWM20=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=j93iNg0aWP450n0a3vmlN7SaoaGxWSkb3BUzwdA2rGcitaFkj5+dSMRAEPgJh7aJl 6ZLAxACbMa3fYYF/mJ/whZO48aCz5FNPlHdLCruy3FguwoIInFxB5BiAJgHZ9q7LUb ufENV/Ga+9Xxe1KeECbeaPU2oA/D0QbA/FPqGaqW5PFGCs4lqTYh6+0lqI7H+HL2pc utgb4ysvYjoWYv23sM/f9OL7I6EfsyQVKEgQnkZ2/YsHRQZJafAIFGKKd6wKdaEObW 4v41gtUlBdI91V2Ku/P1rQLoKXieIFD6k6jS7mV0AINyqNZXXCmLY5VCwGDo5+lmgH tT1ZjcfeH+Pbg== From: "Harry Yoo (Oracle)" Date: Mon, 20 Jul 2026 21:44:34 +0900 Subject: [PATCH slab/for-next v4 5/8] mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260720-kfree_rcu_nolock-v4-5-964e03c41a4e@kernel.org> References: <20260720-kfree_rcu_nolock-v4-0-964e03c41a4e@kernel.org> In-Reply-To: <20260720-kfree_rcu_nolock-v4-0-964e03c41a4e@kernel.org> To: Vlastimil Babka , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Alexei Starovoitov , Andrii Nakryiko , Puranjay Mohan , Amery Hung , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Pedro Falcato , Suren Baghdasaryan , Shengming Hu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, rcu@vger.kernel.org, bpf@vger.kernel.org, "Harry Yoo (Oracle)" X-Mailer: b4 0.14.3 As suggested by Vlastimil Babka [1], kfree_rcu_sheaf() can be used on PREEMPT_RT if we always assume spinning is not allowed on PREEMPT_RT. This is because local_trylock and spinlock_t are safe to use with trylock and unlock as long as the kernel does not spin and the context is not NMI and not hardirq. Now that __kfree_rcu_sheaf() knows how to handle SLAB_FREE_NOLOCK, relax the limitation and try the sheaves path on PREEMPT_RT as well. Keep the lockdep map on non RT kernels. However, do not use the lockdep map on PREEMPT_RT to avoid suppressing valid lockdep warnings. As pointed by Vlastimil Babka [2], on PREEMPT_RT it is unnecessary to defer call_rcu() under IRQ-disabled section or raw spinlock. However, let us avoid adding more complexity as the scenario is not supposed to be common on PREEMPT_RT, with a hope that call_rcu_nolock() will be soon supported in RCU. Link: https://lore.kernel.org/linux-mm/6811cc17-8ee4-48c8-8cbf-6bf4d9f98162@kernel.org [1] Link: https://lore.kernel.org/linux-mm/40591888-3a87-433e-b3d2-cda1cab543be@kernel.org [2] Suggested-by: Vlastimil Babka (SUSE) Signed-off-by: Harry Yoo (Oracle) --- mm/slab_common.c | 12 ++++++++++-- mm/slub.c | 17 ++++++++++------- 2 files changed, 20 insertions(+), 9 deletions(-) diff --git a/mm/slab_common.c b/mm/slab_common.c index 9c9ae1384f47..8c631bf97cd5 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -1595,6 +1595,14 @@ static bool kfree_rcu_sheaf(void *obj) { struct kmem_cache *s; struct slab *slab; + unsigned int free_flags = SLAB_FREE_DEFAULT; + + /* + * It is not safe to spin on PREEMPT_RT because the kernel might be + * holding a raw spinlock and slab acquires sleeping locks. + */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + free_flags = SLAB_FREE_NOLOCK; if (is_vmalloc_addr(obj)) return false; @@ -1605,7 +1613,7 @@ static bool kfree_rcu_sheaf(void *obj) s = slab->slab_cache; if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())) - return __kfree_rcu_sheaf(s, obj, SLAB_FREE_DEFAULT); + return __kfree_rcu_sheaf(s, obj, free_flags); return false; } @@ -1954,7 +1962,7 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr) if (!head) might_sleep(); - if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kfree_rcu_sheaf(ptr)) + if (kfree_rcu_sheaf(ptr)) return; // Queue the object but don't yet schedule the batch. diff --git a/mm/slub.c b/mm/slub.c index 8afa6b47b1f2..deac315d0f23 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -6065,12 +6065,13 @@ static void rcu_free_sheaf(struct rcu_head *head) * kvfree_call_rcu() can be called while holding a raw_spinlock_t. Since * __kfree_rcu_sheaf() may acquire a spinlock_t (sleeping lock on PREEMPT_RT), * this would violate lock nesting rules. Therefore, kvfree_call_rcu() avoids - * this problem by bypassing the sheaves layer entirely on PREEMPT_RT. + * this problem by passing allow_spin = false on PREEMPT_RT. * * However, lockdep still complains that it is invalid to acquire spinlock_t * while holding raw_spinlock_t, even on !PREEMPT_RT where spinlock_t is a * spinning lock. Tell lockdep that acquiring spinlock_t is valid here - * by temporarily raising the wait-type to LD_WAIT_CONFIG. + * by temporarily raising the wait-type to LD_WAIT_CONFIG. Skip the lockdep map + * on PREEMPT_RT to avoid suppressing valid lockdep warnings. */ static DEFINE_WAIT_OVERRIDE_MAP(kfree_rcu_sheaf_map, LD_WAIT_CONFIG); @@ -6080,10 +6081,10 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, unsigned int free_flags) struct slab_sheaf *rcu_sheaf; bool allow_spin = free_flags_allow_spinning(free_flags); - if (WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT))) - return false; + VM_WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT) && allow_spin); - lock_map_acquire_try(&kfree_rcu_sheaf_map); + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + lock_map_acquire_try(&kfree_rcu_sheaf_map); if (!local_trylock(&s->cpu_sheaves->lock)) goto fail; @@ -6181,12 +6182,14 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, unsigned int free_flags) local_unlock(&s->cpu_sheaves->lock); stat(s, FREE_RCU_SHEAF); - lock_map_release(&kfree_rcu_sheaf_map); + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + lock_map_release(&kfree_rcu_sheaf_map); return true; fail: stat(s, FREE_RCU_SHEAF_FAIL); - lock_map_release(&kfree_rcu_sheaf_map); + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + lock_map_release(&kfree_rcu_sheaf_map); return false; } -- 2.53.0