linux-kernel - Re: [REPOST PATCH v4 3/5] kernfs: switch kernfs to use an rwsem

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Date:   Thu, 03 Jun 2021 11:59:26 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Ian Kent <raven@...maw.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>, Eric Sandeen <sandeen@...deen.net>,
        Fox Chen <foxhlchen@...il.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [REPOST PATCH v4 3/5] kernfs: switch kernfs to use an rwsem

Ian Kent <raven@...maw.net> writes:

> The kernfs global lock restricts the ability to perform kernfs node
> lookup operations in parallel during path walks.
>
> Change the kernfs mutex to an rwsem so that, when opportunity arises,
> node searches can be done in parallel with path walk lookups.
>
> Signed-off-by: Ian Kent <raven@...maw.net>
> ---
>  fs/kernfs/dir.c             |  117 ++++++++++++++++++++++++-------------------
>  fs/kernfs/file.c            |    4 +
>  fs/kernfs/inode.c           |   16 +++---
>  fs/kernfs/kernfs-internal.h |    5 +-
>  fs/kernfs/mount.c           |   12 ++--
>  fs/kernfs/symlink.c         |    4 +
>  include/linux/kernfs.h      |    2 -
>  7 files changed, 86 insertions(+), 74 deletions(-)
>
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index 5151c712f06f5..1e2e35a201dfb 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -17,7 +17,7 @@
>  
>  #include "kernfs-internal.h"
>  
> -DEFINE_MUTEX(kernfs_mutex);
> +DECLARE_RWSEM(kernfs_rwsem);
>  static DEFINE_SPINLOCK(kernfs_rename_lock);	/* kn->parent and ->name */
>  static char kernfs_pr_cont_buf[PATH_MAX];	/* protected by rename_lock */
>  static DEFINE_SPINLOCK(kernfs_idr_lock);	/* root->ino_idr */
> @@ -26,10 +26,21 @@ static DEFINE_SPINLOCK(kernfs_idr_lock);	/* root->ino_idr */
>  
>  static bool kernfs_active(struct kernfs_node *kn)
>  {
> -	lockdep_assert_held(&kernfs_mutex);
>  	return atomic_read(&kn->active) >= 0;
>  }
>  
> +static bool kernfs_active_write(struct kernfs_node *kn)
> +{
> +	lockdep_assert_held_write(&kernfs_rwsem);
> +	return kernfs_active(kn);
> +}
> +
> +static bool kernfs_active_read(struct kernfs_node *kn)
> +{
> +	lockdep_assert_held_read(&kernfs_rwsem);
> +	return kernfs_active(kn);
> +}

This bit is unnecessary and confusing.  There is nothing read/write
about how the kernfs file is active (aka being used be a function).
Further all that is needed for correct operation is:

>  static bool kernfs_active(struct kernfs_node *kn)
>  {
> -	lockdep_assert_held(&kernfs_mutex);
> +	lockdep_assert_held(&kernfs_rwsem);
>  	return atomic_read(&kn->active) >= 0;
>  }

Eric