linux-kernel - Re: [PATCH 1/2] kernfs: replace the mutex in kernfs_iop

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Date:   Wed, 2 Dec 2020 13:37:05 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Fox Chen <foxhlchen@...il.com>
Cc:     gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] kernfs: replace the mutex in kernfs_iop_permission
 with a rwlock

On Wed, Dec 02, 2020 at 10:58:36PM +0800, Fox Chen wrote:
> diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
> index 89f6a4214a70..545cdb39b34b 100644
> --- a/include/linux/kernfs.h
> +++ b/include/linux/kernfs.h
> @@ -156,6 +156,7 @@ struct kernfs_node {
>  	unsigned short		flags;
>  	umode_t			mode;
>  	struct kernfs_iattrs	*iattr;
> +	rwlock_t		iattr_rwlock;
>  };

Also, while this might not look like much, kernfs_node is very size
sensitive. There are systems with huge number of these nodes, so I don't
think putting a per-node lock like this is a good idea. Either we can use a
shared iattr protecting lock or play some cmpxchg games when allocating and
setting ->iattr and put the lock there.

Thanks.

-- 
tejun