From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-181.mta0.migadu.com [91.218.175.181]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B5C652F9D82 for ; Sun, 13 Sep 2026 02:15:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.181 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789265712; cv=none; b=YGbcZ9NKReUKQJgqn74vFTMNMwtq+zC0o2N2rMov+Fk6Gf8LVPdM/J88wgeOz+FL9ea39bgLYQQFb7to+UIT/w7zZQJwtpYwKpIJQ9zCViKgq/sbSdEReEHlcH7my31KznNXdeALo/yUwQZyHRuuHwdTsesoim91tBfvJMEjRQU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789265712; c=relaxed/simple; bh=lLMjhtX8mQSkcteJdNHfuS1BULHYOy4nyuYAaZWcCw8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=KQQibXJk/UjOgUW4ubeGShrbWEfkI5mNqoL/ooOsRb8sY/EOVkEotEg50Rcvo1i59EWRutPsVgzBEmIIq8hLjTfBZyGkb4EBTVy1FZy5Q70hoLyIWpHmH+wYq6dehCB45yg4UOi6TpgfZTorudGQm2MKy2gmaVP/e1Si6WHZ4FU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=WfpdAdlW; arc=none smtp.client-ip=91.218.175.181 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="WfpdAdlW" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=lLMjhtX8mQSkcteJdNHfuS1BULHYOy4nyuYAaZWcCw8=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1789265708; v=1; x=1789870508; b=WfpdAdlWBBScYJQwkKUReOvuNHQFT9ZredA9hMe/pb65w2YpBmPzjhUpylw/0CCrT0XylETe VT+/bjsJQZEYBhBGcvXvo/Yrx8ClCXncJXj1AOLK9eHJi+VzhHL57lAydndNZkS4ry1zuqDAgmG VwFeq3iCE66eYNXByJZimixk= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id e3873fdc4d9fcc1f; Sun, 13 Sep 2026 02:15:07 +0000 X-Mizu-Trace-ID: e3873fdc4d9fcc1f X-Migadu-Flow: FLOW_OUT From: Shakeel Butt To: Greg Kroah-Hartman , Tejun Heo , Christian Brauner Cc: Sebastian Andrzej Siewior , Meta kernel team , linux-fsdevel@vger.kernel.org, driver-core@lists.linux.dev, linux-kernel@vger.kernel.org Subject: [PATCH 1/3] kernfs: activate a new node without dropping kernfs_rwsem Date: Sat, 12 Sep 2026 19:14:51 -0700 Message-ID: <20260913021453.21507-2-shakeel.butt@linux.dev> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260913021453.21507-1-shakeel.butt@linux.dev> References: <20260913021453.21507-1-shakeel.butt@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit kernfs_add_one() links the node in, drops the kernfs_rwsem write lock, then calls kernfs_activate(), which takes it again. Roots that do not set KERNFS_ROOT_CREATE_DEACTIVATED, such as sysfs, therefore pay two write locks for every file, directory and symlink created. A new node has no children, so kernfs_activate() would walk only that node. Call kernfs_activate_one() before dropping the lock instead. Its two WARN_ON_ONCE()s still hold: the node was just linked, and nothing can have changed its active count yet. This also closes the window where a node is linked but not yet activated. lock_stat, creating and destroying five dummy netdevs: before after kernfs_rwsem write acquires 1200 830 Assisted-by: LLM Signed-off-by: Shakeel Butt --- fs/kernfs/dir.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index 07abf59f0264..8953e8a07537 100644 --- a/fs/kernfs/dir.c +++ b/fs/kernfs/dir.c @@ -30,6 +30,8 @@ static char kernfs_pr_cont_buf[PATH_MAX]; /* protected by pr_cont_lock */ #define rb_to_kn(X) rb_entry((X), struct kernfs_node, rb) +static void kernfs_activate_one(struct kernfs_node *kn); + static bool __kernfs_active(struct kernfs_node *kn) { return atomic_read(&kn->active) >= 0; @@ -861,7 +863,6 @@ int kernfs_add_one(struct kernfs_node *kn) } up_write(&root->kernfs_iattr_rwsem); - up_write(&root->kernfs_rwsem); /* * Activate the new node unless CREATE_DEACTIVATED is requested. @@ -869,9 +870,15 @@ int kernfs_add_one(struct kernfs_node *kn) * activating the node with kernfs_activate(). A node which hasn't * been activated is not visible to userland and its removal won't * trigger deactivation. + * + * @kn has no children yet, so kernfs_activate() would walk only @kn. + * Do it here rather than dropping the write lock and taking it again + * for every new node. */ - if (!(kernfs_root(kn)->flags & KERNFS_ROOT_CREATE_DEACTIVATED)) - kernfs_activate(kn); + if (!(root->flags & KERNFS_ROOT_CREATE_DEACTIVATED)) + kernfs_activate_one(kn); + + up_write(&root->kernfs_rwsem); return 0; out_unlock: -- 2.53.0-Meta