linux-kernel - [PATCH v2 4/4] kernfs: add warning about implementing freeze/thaw

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Message-ID: <20250402-work-freeze-v2-4-6719a97b52ac@kernel.org>
Date: Wed,  2 Apr 2025 16:07:34 +0200
From: Christian Brauner <brauner@...nel.org>
To: linux-fsdevel@...r.kernel.org,
	jack@...e.cz
Cc: Christian Brauner <brauner@...nel.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	mcgrof@...nel.org,
	hch@...radead.org,
	david@...morbit.com,
	rafael@...nel.org,
	djwong@...nel.org,
	pavel@...nel.org,
	peterz@...radead.org,
	mingo@...hat.com,
	will@...nel.org,
	boqun.feng@...il.com
Subject: [PATCH v2 4/4] kernfs: add warning about implementing freeze/thaw

Sysfs is built on top of kernfs and sysfs provides the power management
infrastructure to support suspend/hibernate by writing to various files
in /sys/power/. As filesystems may be automatically frozen during
suspend/hibernate implementing freeze/thaw support for kernfs
generically will cause deadlocks as the suspending/hibernation
initiating task will hold a VFS lock that it will then wait upon to be
released. If freeze/thaw for kernfs is needed talk to the VFS.

Signed-off-by: Christian Brauner <brauner@...nel.org>
---
 fs/kernfs/mount.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 1358c21837f1..d2073bb2b633 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -62,6 +62,21 @@ const struct super_operations kernfs_sops = {
 
 	.show_options	= kernfs_sop_show_options,
 	.show_path	= kernfs_sop_show_path,
+
+	/*
+	 * sysfs is built on top of kernfs and sysfs provides the power
+	 * management infrastructure to support suspend/hibernate by
+	 * writing to various files in /sys/power/. As filesystems may
+	 * be automatically frozen during suspend/hibernate implementing
+	 * freeze/thaw support for kernfs generically will cause
+	 * deadlocks as the suspending/hibernation initiating task will
+	 * hold a VFS lock that it will then wait upon to be released.
+	 * If freeze/thaw for kernfs is needed talk to the VFS.
+	 */
+	.freeze_fs	= NULL,
+	.unfreeze_fs	= NULL,
+	.freeze_super	= NULL,
+	.thaw_super	= NULL,
 };
 
 static int kernfs_encode_fh(struct inode *inode, __u32 *fh, int *max_len,

-- 
2.47.2