From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 400B4C47E48 for ; Thu, 15 Jul 2021 13:33:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1EAAE613CC for ; Thu, 15 Jul 2021 13:33:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235859AbhGONgY (ORCPT ); Thu, 15 Jul 2021 09:36:24 -0400 Received: from mail.kernel.org ([198.145.29.99]:51174 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233957AbhGONgY (ORCPT ); Thu, 15 Jul 2021 09:36:24 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id B5493613C4; Thu, 15 Jul 2021 13:33:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1626356011; bh=IVP/s33scCUhAITuuMBnyFZ3AUfIr+z6Xyu4lroBTN0=; h=Subject:To:Cc:From:Date:From; b=lV9ctHL82d28Qp827O/fEE3dT82k1w3rZT79D5QOKwfFWa/5HM2XAGoWJy61BtUxw emsh4Kg0+gCYYXp0SfTF7gRmFDg/WhA7vhtFPXFD2/+5wDxlqH61tAuSm8xa4Gjo2m et+VpWN5DU0jtB1RR5LlpdkA1+oaPlrp/mLsUXRg= Subject: FAILED: patch "[PATCH] xfrm: policy: Read seqcount outside of rcu-read side in" failed to apply to 4.14-stable tree To: varad.gautam@suse.com, a.darwish@linutronix.de, davem@davemloft.net, fw@strlen.de, herbert@gondor.apana.org.au, kuba@kernel.org, linux-rt-users@vger.kernel.org, steffen.klassert@secunet.com Cc: From: Date: Thu, 15 Jul 2021 15:33:21 +0200 Message-ID: <162635600118223@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org The patch below does not apply to the 4.14-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From d7b0408934c749f546b01f2b33d07421a49b6f3e Mon Sep 17 00:00:00 2001 From: Varad Gautam Date: Fri, 28 May 2021 18:04:06 +0200 Subject: [PATCH] xfrm: policy: Read seqcount outside of rcu-read side in xfrm_policy_lookup_bytype xfrm_policy_lookup_bytype loops on seqcount mutex xfrm_policy_hash_generation within an RCU read side critical section. Although ill advised, this is fine if the loop is bounded. xfrm_policy_hash_generation wraps mutex hash_resize_mutex, which is used to serialize writers (xfrm_hash_resize, xfrm_hash_rebuild). This is fine too. On PREEMPT_RT=y, the read_seqcount_begin call within xfrm_policy_lookup_bytype emits a mutex lock/unlock for hash_resize_mutex. Mutex locking is fine, since RCU read side critical sections are allowed to sleep with PREEMPT_RT. xfrm_hash_resize can, however, block on synchronize_rcu while holding hash_resize_mutex. This leads to the following situation on PREEMPT_RT, where the writer is blocked on RCU grace period expiry, while the reader is blocked on a lock held by the writer: Thead 1 (xfrm_hash_resize) Thread 2 (xfrm_policy_lookup_bytype) rcu_read_lock(); mutex_lock(&hash_resize_mutex); read_seqcount_begin(&xfrm_policy_hash_generation); mutex_lock(&hash_resize_mutex); // block xfrm_bydst_resize(); synchronize_rcu(); // block Move the read_seqcount_begin call outside of the RCU read side critical section, and do an rcu_read_unlock/retry if we got stale data within the critical section. On non-PREEMPT_RT, this shortens the time spent within RCU read side critical section in case the seqcount needs a retry, and avoids unbounded looping. Fixes: 77cc278f7b20 ("xfrm: policy: Use sequence counters with associated lock") Signed-off-by: Varad Gautam Cc: linux-rt-users Cc: netdev@vger.kernel.org Cc: stable@vger.kernel.org # v4.9 Cc: Steffen Klassert Cc: Herbert Xu Cc: "David S. Miller" Cc: Jakub Kicinski Cc: Florian Westphal Cc: "Ahmed S. Darwish" Signed-off-by: Steffen Klassert Acked-by: Ahmed S. Darwish diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c index b74f28cabe24..8c56e3e59c3c 100644 --- a/net/xfrm/xfrm_policy.c +++ b/net/xfrm/xfrm_policy.c @@ -2092,12 +2092,15 @@ static struct xfrm_policy *xfrm_policy_lookup_bytype(struct net *net, u8 type, if (unlikely(!daddr || !saddr)) return NULL; - rcu_read_lock(); retry: - do { - sequence = read_seqcount_begin(&xfrm_policy_hash_generation); - chain = policy_hash_direct(net, daddr, saddr, family, dir); - } while (read_seqcount_retry(&xfrm_policy_hash_generation, sequence)); + sequence = read_seqcount_begin(&xfrm_policy_hash_generation); + rcu_read_lock(); + + chain = policy_hash_direct(net, daddr, saddr, family, dir); + if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence)) { + rcu_read_unlock(); + goto retry; + } ret = NULL; hlist_for_each_entry_rcu(pol, chain, bydst) { @@ -2128,11 +2131,15 @@ static struct xfrm_policy *xfrm_policy_lookup_bytype(struct net *net, u8 type, } skip_inexact: - if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence)) + if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence)) { + rcu_read_unlock(); goto retry; + } - if (ret && !xfrm_pol_hold_rcu(ret)) + if (ret && !xfrm_pol_hold_rcu(ret)) { + rcu_read_unlock(); goto retry; + } fail: rcu_read_unlock();