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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BAC25C25B06 for ; Thu, 11 Aug 2022 16:37:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236589AbiHKQhj (ORCPT ); Thu, 11 Aug 2022 12:37:39 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38284 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239082AbiHKQgT (ORCPT ); Thu, 11 Aug 2022 12:36:19 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A4DC2816BB; Thu, 11 Aug 2022 09:11:40 -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 ams.source.kernel.org (Postfix) with ESMTPS id 42E22B82164; Thu, 11 Aug 2022 16:11:39 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1F976C433B5; Thu, 11 Aug 2022 16:11:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1660234298; bh=utghrLW32ISiREkGWf4NjEJsgWU4hb+gpkwwBKIRt1c=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ogN8qujRzQ9XovTzY+lt86EUk+ZVheOUIWJUoZgmxfiKNkC6OYVDb0EsozBQhQiF8 c5Y1A3iXr1yXu5+G8DzoNOq2/wQAT/NO/7ErSggXMjNDDqYBSFsWWOomMgyK/TG8UM xD9vUgiyBzmbjfkX6v5kaA427QZS9NU3TotxfqCAo95etfQGrbK6Yj8iPXsKntB4Ho 1NyEfewZllss/AogobOY07qqFE21wxqz9C/G9i7mNO0oBjpg5ksCr9O9+3ujc3ctn4 MLTg7xnT3bEB6rxqJQ+zOLvJk41sWL7ZgAVGRHJDcKCBb9MnMINHJGbavwjvnkmw9r GWQgSRTHfReKQ== From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Sebastian Andrzej Siewior , Oleg.Karfich@wago.com, Thomas Gleixner , Al Viro , Sasha Levin , linux-fsdevel@vger.kernel.org Subject: [PATCH AUTOSEL 4.14 14/14] fs/dcache: Disable preemption on i_dir_seq write side on PREEMPT_RT Date: Thu, 11 Aug 2022 12:10:43 -0400 Message-Id: <20220811161050.1543183-14-sashal@kernel.org> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220811161050.1543183-1-sashal@kernel.org> References: <20220811161050.1543183-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Sebastian Andrzej Siewior [ Upstream commit cf634d540a29018e8d69ab1befb7e08182bc6594 ] i_dir_seq is a sequence counter with a lock which is represented by the lowest bit. The writer atomically updates the counter which ensures that it can be modified by only one writer at a time. This requires preemption to be disabled across the write side critical section. On !PREEMPT_RT kernels this is implicit by the caller acquiring dentry::lock. On PREEMPT_RT kernels spin_lock() does not disable preemption which means that a preempting writer or reader would live lock. It's therefore required to disable preemption explicitly. An alternative solution would be to replace i_dir_seq with a seqlock_t for PREEMPT_RT, but that comes with its own set of problems due to arbitrary lock nesting. A pure sequence count with an associated spinlock is not possible because the locks held by the caller are not necessarily related. As the critical section is small, disabling preemption is a sensible solution. Reported-by: Oleg.Karfich@wago.com Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Thomas Gleixner Signed-off-by: Sebastian Andrzej Siewior Link: https://lkml.kernel.org/r/20220613140712.77932-2-bigeasy@linutronix.de Signed-off-by: Al Viro Signed-off-by: Sasha Levin --- fs/dcache.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/fs/dcache.c b/fs/dcache.c index e12246378834..faf6ca33fd51 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -2455,7 +2455,15 @@ EXPORT_SYMBOL(d_rehash); static inline unsigned start_dir_add(struct inode *dir) { - + /* + * The caller holds a spinlock (dentry::d_lock). On !PREEMPT_RT + * kernels spin_lock() implicitly disables preemption, but not on + * PREEMPT_RT. So for RT it has to be done explicitly to protect + * the sequence count write side critical section against a reader + * or another writer preempting, which would result in a live lock. + */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_disable(); for (;;) { unsigned n = dir->i_dir_seq; if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n) @@ -2467,6 +2475,8 @@ static inline unsigned start_dir_add(struct inode *dir) static inline void end_dir_add(struct inode *dir, unsigned n) { smp_store_release(&dir->i_dir_seq, n + 2); + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_enable(); } static void d_wait_lookup(struct dentry *dentry) -- 2.35.1