From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 11E8E38CFE6 for ; Sat, 28 Feb 2026 18:08:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772302121; cv=none; b=KQ0MUfx2RQZe7cujRU6W6HEyw4E7f/nMeskjHR9rOHXEvvyjEvMgB3uBbKaxxE8CffIeb3YrrcbrCifThvp3QpsA0nGS3hVDehD14mgAeZYa1KwMA83CuBdfEoIirtTjoR5BwWi80Y6qa/ruaTLdNdtfUw+uLcDVTskMwiYVX7c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772302121; c=relaxed/simple; bh=vCjFclKE2L4E4kyR9zC62v7nq0sPh6EHH8479x7zdQU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=eJoBABzoFXrGpSICec1GXQ1ReSQ2+6GNRGQGjQdqk7YZdYu7hGJOLj6Ql3/3odXgxXlO5JidPmJb83koFMOw/ohRS3HpzQ9Kp5gkoL5821cat/MpbnL5HYRiF1UF9+eXdhhdDtdq4Xu6W9Usp4/sjaBT3Z0QPPGnH0Gx4Jx8jfc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=CSt6goH5; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="CSt6goH5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 758F4C2BC87; Sat, 28 Feb 2026 18:08:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772302121; bh=vCjFclKE2L4E4kyR9zC62v7nq0sPh6EHH8479x7zdQU=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=CSt6goH5yvGhy2RGbwXm1tpyQqKulOhga3OElC1NNtdTIgAwdZAo+9GSDoFRhTw0r Nd/1GyIJAanM5+demETrYZbXU63/wit/Ltbd72bOdUrP+u4WayTBQaTb5so8oVNNGs Y+U/iETeMc9LXn6ahBUlrAoDmejeDTyLiKjOTFLN6HvFX7g6AB06e53UqOs5M1jDM2 f//1rsFkqXfDRaf3Ilk4WrfAGJ5ncyhqFRfMvYhuo8CDG14jYKlRcPTEeBKQiv+l4B A1KRgrZvd6gkWgBm6z5t2ncOBD5HXLcwiBueJw0RTDlUcOwT9+0uIsYLwaD0N1ZVXu D0n6/GrN4+kgA== From: Sasha Levin To: patches@lists.linux.dev Cc: Abhishek Bapat , Jan Kara , Sasha Levin Subject: [PATCH 6.6 111/283] quota: fix livelock between quotactl and freeze_super Date: Sat, 28 Feb 2026 13:04:13 -0500 Message-ID: <20260228180709.1583486-111-sashal@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260228180709.1583486-1-sashal@kernel.org> References: <20260228180709.1583486-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore Content-Transfer-Encoding: 8bit From: Abhishek Bapat [ Upstream commit 77449e453dfc006ad738dec55374c4cbc056fd39 ] When a filesystem is frozen, quotactl_block() enters a retry loop waiting for the filesystem to thaw. It acquires s_umount, checks the freeze state, drops s_umount and uses sb_start_write() - sb_end_write() pair to wait for the unfreeze. However, this retry loop can trigger a livelock issue, specifically on kernels with preemption disabled. The mechanism is as follows: 1. freeze_super() sets SB_FREEZE_WRITE and calls sb_wait_write(). 2. sb_wait_write() calls percpu_down_write(), which initiates synchronize_rcu(). 3. Simultaneously, quotactl_block() spins in its retry loop, immediately executing the sb_start_write() - sb_end_write() pair. 4. Because the kernel is non-preemptible and the loop contains no scheduling points, quotactl_block() never yields the CPU. This prevents that CPU from reaching an RCU quiescent state. 5. synchronize_rcu() in the freezer thread waits indefinitely for the quotactl_block() CPU to report a quiescent state. 6. quotactl_block() spins indefinitely waiting for the freezer to advance, which it cannot do as it is blocked on the RCU sync. This results in a hang of the freezer process and 100% CPU usage by the quota process. While this can occur intermittently on multi-core systems, it is reliably reproducing on a node with the following script, running both the freezer and the quota toggle on the same CPU: # mkfs.ext4 -O quota /dev/sda 2g && mkdir a_mount # mount /dev/sda -o quota,usrquota,grpquota a_mount # taskset -c 3 bash -c "while true; do xfs_freeze -f a_mount; \ xfs_freeze -u a_mount; done" & # taskset -c 3 bash -c "while true; do quotaon a_mount; \ quotaoff a_mount; done" & Adding cond_resched() to the retry loop fixes the issue. It acts as an RCU quiescent state, allowing synchronize_rcu() in percpu_down_write() to complete. Fixes: 576215cffdef ("fs: Drop wait_unfrozen wait queue") Signed-off-by: Abhishek Bapat Link: https://patch.msgid.link/20260115213103.1089129-1-abhishekbapat@google.com Signed-off-by: Jan Kara Signed-off-by: Sasha Levin --- fs/quota/quota.c | 1 + 1 file changed, 1 insertion(+) diff --git a/fs/quota/quota.c b/fs/quota/quota.c index 0e41fb84060f5..5be53cae2c95d 100644 --- a/fs/quota/quota.c +++ b/fs/quota/quota.c @@ -899,6 +899,7 @@ static struct super_block *quotactl_block(const char __user *special, int cmd) sb_start_write(sb); sb_end_write(sb); put_super(sb); + cond_resched(); goto retry; } return sb; -- 2.51.0