From patchwork Wed May 10 17:06:04 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Paul E. McKenney" X-Patchwork-Id: 13237100 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3FB20C7EE22 for ; Wed, 10 May 2023 17:06:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236003AbjEJRG0 (ORCPT ); Wed, 10 May 2023 13:06:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49816 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236367AbjEJRGX (ORCPT ); Wed, 10 May 2023 13:06:23 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8ABD6358C; Wed, 10 May 2023 10:06:11 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 1FE75636DF; Wed, 10 May 2023 17:06:11 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7F747C433EF; Wed, 10 May 2023 17:06:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1683738370; bh=kQwhUcHnUV1oB8K+SYCdQwdWT6mtPz7OHiOwXCmWFXs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Anbm9VOAYtEtoX7qGLRyHb8ln3njJ14N8ou716QX9jLtd0EpbHyNlc2Ij8OpNmEPC JfpGlBBiSXXmTdoJMZpLaSTmod4JNdZFsA1Ok2UZhRgVbN2DRHUNBj9fNr0kb3KYg9 i3Csz5mZgQ5uogU9/JBg3+gQp/LAULludnnBBF6OSjDCl8M92XV9w//2ZWovyGFt3o STHmsrW6Cq2+bTeTAQE95svezqRBeyWRELebPS1TNyPVnLK0MIkhw/nM5Jc0nrszke 9Vcc7J+uTIlgoKD7UFBqjIq3UCbr9XwsW4kqeqd6NFM9hjW6Qpa4tOLLlpNCgnFjlz PkGcd1uMMKl5A== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 2BB0ECE126E; Wed, 10 May 2023 10:06:10 -0700 (PDT) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, Frederic Weisbecker , "Paul E . McKenney" Subject: [PATCH rcu 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading Date: Wed, 10 May 2023 10:06:04 -0700 Message-Id: <20230510170607.2187978-1-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: <9c5fae22-e25e-4548-b6e9-117495b29e62@paulmck-laptop> References: <9c5fae22-e25e-4548-b6e9-117495b29e62@paulmck-laptop> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org From: Frederic Weisbecker The shrinker may run concurrently with callbacks (de-)offloading. As such, calling rcu_nocb_lock() is very dangerous because it does a conditional locking. The worst outcome is that rcu_nocb_lock() doesn't lock but rcu_nocb_unlock() eventually unlocks, or the reverse, creating an imbalance. Fix this with protecting against (de-)offloading using the barrier mutex. Although if the barrier mutex is contended, which should be rare, then step aside so as not to trigger a mutex VS allocation dependency chain. Signed-off-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney --- kernel/rcu/tree_nocb.h | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index f2280616f9d5..1a86883902ce 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -1336,13 +1336,33 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) unsigned long flags; unsigned long count = 0; + /* + * Protect against concurrent (de-)offloading. Otherwise nocb locking + * may be ignored or imbalanced. + */ + if (!mutex_trylock(&rcu_state.barrier_mutex)) { + /* + * But really don't insist if barrier_mutex is contended since we + * can't guarantee that it will never engage in a dependency + * chain involving memory allocation. The lock is seldom contended + * anyway. + */ + return 0; + } + /* Snapshot count of all CPUs */ for_each_possible_cpu(cpu) { struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); - int _count = READ_ONCE(rdp->lazy_len); + int _count; + + if (!rcu_rdp_is_offloaded(rdp)) + continue; + + _count = READ_ONCE(rdp->lazy_len); if (_count == 0) continue; + rcu_nocb_lock_irqsave(rdp, flags); WRITE_ONCE(rdp->lazy_len, 0); rcu_nocb_unlock_irqrestore(rdp, flags); @@ -1352,6 +1372,9 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) if (sc->nr_to_scan <= 0) break; } + + mutex_unlock(&rcu_state.barrier_mutex); + return count ? count : SHRINK_STOP; }