From nobody Thu May 7 23:11:04 2026 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 74540C433EF for ; Mon, 16 May 2022 18:29:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344939AbiEPS3l (ORCPT ); Mon, 16 May 2022 14:29:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53122 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1343520AbiEPS33 (ORCPT ); Mon, 16 May 2022 14:29:29 -0400 Received: from mail-yb1-xb4a.google.com (mail-yb1-xb4a.google.com [IPv6:2607:f8b0:4864:20::b4a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 83F573E0E5 for ; Mon, 16 May 2022 11:29:28 -0700 (PDT) Received: by mail-yb1-xb4a.google.com with SMTP id z39-20020a25a12a000000b0064dbcd526ffso2279305ybh.15 for ; Mon, 16 May 2022 11:29:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:message-id:mime-version:subject:from:to:cc; bh=CGsBgLCZ1E7ckC4MxiSy9sirQYDPuKObKimIDda5O9I=; b=VIcEg1aulc0rZskxO6n7AgH4wUYDZgUEC7oUOu3scKOOGZb8FeXCoRIkU/vsRrjNHh DlRqvZOUuhfgzMsvWfiCJyK2mNmrjOCc9VHGEzAD1kccaZMQZ9yPZkMLyH2RzDw3LNzX 1PBhU4yJNowUHFBtlK3Ed0W1PJyndEH8VLNEHtwAr1WrC87xMlfp71wyzTImBu/j3Ucc cjn63FrlMgecWpoYTIuXzeUwMP2SNrdLRMZCvJcatCfyTFHhJvyFxjYex8589wfzjpaq gHyndZDEwQNQ60gC1bw2mhWXwmKcO5aZl+Tw6ivMKFCTZOiNj+SVjDPFuZPVeWjxIfkl cCEg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=CGsBgLCZ1E7ckC4MxiSy9sirQYDPuKObKimIDda5O9I=; b=XiAnxzuriLIJT5Wf3WOzAlJ/u8RECcOEWNLJ1a/vNxjMCJk8hinJCw0K7rQwF7TVfP zYVIhp1+3Sf8gm6onbl1xNrprY6ssxmwXI1dIxBNgY72oqwf6hHN18yyktEwppqHxJa1 HzDONgMQoKpLd/piMRWo1WNhVAGN+bWk9etWicoIyTmfF22CkZp+JutXKGlCDsbPZbZx csfdWucIBfeCQFvxOa4YU2aZWocwWtJtXcNRBuNErPCs80ghZz4S/atpkfi11byD3fa/ /3EtvUGo2ztJtx7GVoDqCCUO8TkjXzIzSJV16g9ENVTUTkmIZ0yhGMyJ7Wm+5iFDNxtl xG+w== X-Gm-Message-State: AOAM531YUPSFf7obdM9hUzHHfPEVtdyamJt2TTcVD4CiJocMcxlXCOSk hXTw8Cty537vH7ca4i7btyIMoLhJpIk= X-Google-Smtp-Source: ABdhPJyjtL3SjX2c2EVY0p5QZbSrJrTJPhvHli/Dz5nPlQrg28kyyyPOGyA7ksdmcRkzjA3j43SkwwqK0fA= X-Received: from haoluo.svl.corp.google.com ([2620:15c:2cd:202:e952:d4d4:b0de:c336]) (user=haoluo job=sendgmr) by 2002:a81:2084:0:b0:2fb:7a14:ea36 with SMTP id g126-20020a812084000000b002fb7a14ea36mr21166392ywg.26.1652725767645; Mon, 16 May 2022 11:29:27 -0700 (PDT) Date: Mon, 16 May 2022 11:28:59 -0700 Message-Id: <20220516182859.3131675-1-haoluo@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.36.1.124.g0e6072fb45-goog Subject: [PATCH] kernfs: Separate kernfs_pr_cont_buf and rename_lock. From: Hao Luo To: Greg Kroah-Hartman , Tejun Heo Cc: linux-kernel@vger.kernel.org, Hao Luo Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Previously the protection of kernfs_pr_cont_buf was piggy backed by rename_lock, which means that pr_cont() needs to be protected under rename_lock. This can cause potential circular lock dependencies. If there is an OOM, we have the following call hierarchy: -> cpuset_print_current_mems_allowed() -> pr_cont_cgroup_name() -> pr_cont_kernfs_name() pr_cont_kernfs_name() will grab rename_lock and call printk. So we have the following lock dependencies: kernfs_rename_lock -> console_sem Sometimes, printk does a wakeup before releasing console_sem, which has the dependence chain: console_sem -> p->pi_lock -> rq->lock Now, imagine one wants to read cgroup_name under rq->lock, for example, printing cgroup_name in a tracepoint in the scheduler code. They will be holding rq->lock and take rename_lock: rq->lock -> kernfs_rename_lock Now they will deadlock. A prevention to this circular lock dependency is to separate the protection of pr_cont_buf from rename_lock. In principle, rename_lock is to protect the integrity of cgroup name when copying to buf. Once pr_cont_buf has got its content, rename_lock can be dropped. So it's safe to drop rename_lock after kernfs_name_locked (and kernfs_path_from_node_locked) and rely on a dedicated pr_cont_lock to protect pr_cont_buf. Signed-off-by: Hao Luo Acked-by: Tejun Heo --- fs/kernfs/dir.c | 24 ++++++++++++------------ 1 file changed, 12 insertions(+), 12 deletions(-) diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index e205fde7163a..966d24562f0f 100644 --- a/fs/kernfs/dir.c +++ b/fs/kernfs/dir.c @@ -18,7 +18,8 @@ #include "kernfs-internal.h" =20 static DEFINE_SPINLOCK(kernfs_rename_lock); /* kn->parent and ->name */ -static char kernfs_pr_cont_buf[PATH_MAX]; /* protected by rename_lock */ +static DEFINE_SPINLOCK(kernfs_pr_cont_lock); +static char kernfs_pr_cont_buf[PATH_MAX]; /* protected by pr_cont_lock */ static DEFINE_SPINLOCK(kernfs_idr_lock); /* root->ino_idr */ =20 #define rb_to_kn(X) rb_entry((X), struct kernfs_node, rb) @@ -229,12 +230,12 @@ void pr_cont_kernfs_name(struct kernfs_node *kn) { unsigned long flags; =20 - spin_lock_irqsave(&kernfs_rename_lock, flags); + spin_lock_irqsave(&kernfs_pr_cont_lock, flags); =20 - kernfs_name_locked(kn, kernfs_pr_cont_buf, sizeof(kernfs_pr_cont_buf)); + kernfs_name(kn, kernfs_pr_cont_buf, sizeof(kernfs_pr_cont_buf)); pr_cont("%s", kernfs_pr_cont_buf); =20 - spin_unlock_irqrestore(&kernfs_rename_lock, flags); + spin_unlock_irqrestore(&kernfs_pr_cont_lock, flags); } =20 /** @@ -248,10 +249,10 @@ void pr_cont_kernfs_path(struct kernfs_node *kn) unsigned long flags; int sz; =20 - spin_lock_irqsave(&kernfs_rename_lock, flags); + spin_lock_irqsave(&kernfs_pr_cont_lock, flags); =20 - sz =3D kernfs_path_from_node_locked(kn, NULL, kernfs_pr_cont_buf, - sizeof(kernfs_pr_cont_buf)); + sz =3D kernfs_path_from_node(kn, NULL, kernfs_pr_cont_buf, + sizeof(kernfs_pr_cont_buf)); if (sz < 0) { pr_cont("(error)"); goto out; @@ -265,7 +266,7 @@ void pr_cont_kernfs_path(struct kernfs_node *kn) pr_cont("%s", kernfs_pr_cont_buf); =20 out: - spin_unlock_irqrestore(&kernfs_rename_lock, flags); + spin_unlock_irqrestore(&kernfs_pr_cont_lock, flags); } =20 /** @@ -823,13 +824,12 @@ static struct kernfs_node *kernfs_walk_ns(struct kern= fs_node *parent, =20 lockdep_assert_held_read(&kernfs_root(parent)->kernfs_rwsem); =20 - /* grab kernfs_rename_lock to piggy back on kernfs_pr_cont_buf */ - spin_lock_irq(&kernfs_rename_lock); + spin_lock_irq(&kernfs_pr_cont_lock); =20 len =3D strlcpy(kernfs_pr_cont_buf, path, sizeof(kernfs_pr_cont_buf)); =20 if (len >=3D sizeof(kernfs_pr_cont_buf)) { - spin_unlock_irq(&kernfs_rename_lock); + spin_unlock_irq(&kernfs_pr_cont_lock); return NULL; } =20 @@ -841,7 +841,7 @@ static struct kernfs_node *kernfs_walk_ns(struct kernfs= _node *parent, parent =3D kernfs_find_ns(parent, name, ns); } =20 - spin_unlock_irq(&kernfs_rename_lock); + spin_unlock_irq(&kernfs_pr_cont_lock); =20 return parent; } --=20 2.36.1.124.g0e6072fb45-goog