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

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Message-ID: <arftvbus4knelyjz5htjdm77fqjalv2haeozfkuxdvyipuge52@wbnzvhdulu25>
Date: Thu, 3 Apr 2025 17:00:28 +0200
From: Jan Kara <jack@...e.cz>
To: Christian Brauner <brauner@...nel.org>
Cc: linux-fsdevel@...r.kernel.org, jack@...e.cz, 
	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: Re: [PATCH v2 4/4] kernfs: add warning about implementing freeze/thaw

On Wed 02-04-25 16:07:34, Christian Brauner wrote:
> 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>

Yeah, good idea. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  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
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR