filelock: new helper: vfs_inode_has_locks

Ceph has a need to know whether a particular inode has any locks set on
it. It's currently tracking that by a num_locks field in its
filp->private_data, but that's problematic as it tries to decrement this
field when releasing locks and that can race with the file being torn
down.

Add a new vfs_inode_has_locks helper that just returns whether any locks
are currently held on the inode.

Cc: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/locks.c         | 23 +++++++++++++++++++++++
 include/linux/fs.h |  1 +
 2 files changed, 24 insertions(+)

Jeff,

I have tested this and it worked very well for ceph.

Tested-by: Xiubo Li <xiubli@redhat.com>

Thanks!

- Xiubo

On 16/11/2022 19:26, Jeff Layton wrote:
> Ceph has a need to know whether a particular inode has any locks set on
> it. It's currently tracking that by a num_locks field in its
> filp->private_data, but that's problematic as it tries to decrement this
> field when releasing locks and that can race with the file being torn
> down.
>
> Add a new vfs_inode_has_locks helper that just returns whether any locks
> are currently held on the inode.
>
> Cc: Xiubo Li <xiubli@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@infradead.org>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>   fs/locks.c         | 23 +++++++++++++++++++++++
>   include/linux/fs.h |  1 +
>   2 files changed, 24 insertions(+)
>
> diff --git a/fs/locks.c b/fs/locks.c
> index 5876c8ff0edc..9ccf89b6c95d 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -2672,6 +2672,29 @@ int vfs_cancel_lock(struct file *filp, struct file_lock *fl)
>   }
>   EXPORT_SYMBOL_GPL(vfs_cancel_lock);
>   
> +/**
> + * vfs_inode_has_locks - are any file locks held on @inode?
> + * @inode: inode to check for locks
> + *
> + * Return true if there are any FL_POSIX or FL_FLOCK locks currently
> + * set on @inode.
> + */
> +bool vfs_inode_has_locks(struct inode *inode)
> +{
> +	struct file_lock_context *ctx;
> +	bool ret;
> +
> +	ctx = smp_load_acquire(&inode->i_flctx);
> +	if (!ctx)
> +		return false;
> +
> +	spin_lock(&ctx->flc_lock);
> +	ret = !list_empty(&ctx->flc_posix) || !list_empty(&ctx->flc_flock);
> +	spin_unlock(&ctx->flc_lock);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(vfs_inode_has_locks);
> +
>   #ifdef CONFIG_PROC_FS
>   #include <linux/proc_fs.h>
>   #include <linux/seq_file.h>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index e654435f1651..d6cb42b7e91c 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1170,6 +1170,7 @@ extern int locks_delete_block(struct file_lock *);
>   extern int vfs_test_lock(struct file *, struct file_lock *);
>   extern int vfs_lock_file(struct file *, unsigned int, struct file_lock *, struct file_lock *);
>   extern int vfs_cancel_lock(struct file *filp, struct file_lock *fl);
> +bool vfs_inode_has_locks(struct inode *inode);
>   extern int locks_lock_inode_wait(struct inode *inode, struct file_lock *fl);
>   extern int __break_lease(struct inode *inode, unsigned int flags, unsigned int type);
>   extern void lease_get_mtime(struct inode *, struct timespec64 *time);