Re: [PATCH RFC 1/3] fs/lock: add new callback, lm_expire_lock, to lock_manager_operations

[Jeff Layton <jlayton@redhat.com>]

On Fri, 2022-01-28 at 11:39 -0800, Dai Ngo wrote:
> Add new callback, lm_expire_lock, to lock_manager_operations to allow
> the lock manager to take appropriate action to resolve the lock conflict
> if possible. The callback takes 1 argument, the file_lock of the blocker
> and returns true if the conflict was resolved else returns false. Note
> that the lock manager has to be able to resolve the conflict while
> the spinlock flc_lock is held.
> 
> Lock manager, such as NFSv4 courteous server, uses this callback to
> resolve conflict by destroying lock owner, or the NFSv4 courtesy client
> (client that has expired but allowed to maintains its states) that owns
> the lock.
> 
> Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
> ---
>  Documentation/filesystems/locking.rst |  2 ++
>  fs/locks.c                            | 14 ++++++++++----
>  include/linux/fs.h                    |  1 +
>  3 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
> index d36fe79167b3..57ce0fbc8ab1 100644
> --- a/Documentation/filesystems/locking.rst
> +++ b/Documentation/filesystems/locking.rst
> @@ -439,6 +439,7 @@ prototypes::
>  	void (*lm_break)(struct file_lock *); /* break_lease callback */
>  	int (*lm_change)(struct file_lock **, int);
>  	bool (*lm_breaker_owns_lease)(struct file_lock *);
> +	bool (*lm_lock_conflict)(struct file_lock *);
>  
>  locking rules:
>  
> @@ -450,6 +451,7 @@ lm_grant:		no		no			no
>  lm_break:		yes		no			no
>  lm_change		yes		no			no
>  lm_breaker_owns_lease:	no		no			no
> +lm_lock_conflict:       no		no			no
>  ======================	=============	=================	=========
>  
>  buffer_head
> diff --git a/fs/locks.c b/fs/locks.c
> index 0fca9d680978..052b42cc7f25 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -853,10 +853,13 @@ posix_test_lock(struct file *filp, struct file_lock *fl)
>  
>  	spin_lock(&ctx->flc_lock);
>  	list_for_each_entry(cfl, &ctx->flc_posix, fl_list) {
> -		if (posix_locks_conflict(fl, cfl)) {
> -			locks_copy_conflock(fl, cfl);
> -			goto out;
> -		}
> +		if (!posix_locks_conflict(fl, cfl))
> +			continue;
> +		if (cfl->fl_lmops && cfl->fl_lmops->lm_lock_conflict &&
> +			!cfl->fl_lmops->lm_lock_conflict(cfl))
> +			continue;
> +		locks_copy_conflock(fl, cfl);
> +		goto out;
>  	}
>  	fl->fl_type = F_UNLCK;
>  out:
> @@ -1059,6 +1062,9 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
>  		list_for_each_entry(fl, &ctx->flc_posix, fl_list) {
>  			if (!posix_locks_conflict(request, fl))
>  				continue;
> +			if (fl->fl_lmops && fl->fl_lmops->lm_lock_conflict &&
> +				!fl->fl_lmops->lm_lock_conflict(fl))
> +				continue;

The naming of this op is a little misleading. We already know that there
is a lock confict in this case. The question is whether it's resolvable
by expiring a tardy client. That said, I don't have a better name to
suggest at the moment.

A comment about what this function actually tells us would be nice here.

>  			if (conflock)
>  				locks_copy_conflock(conflock, fl);
>  			error = -EAGAIN;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index bbf812ce89a8..21cb7afe2d63 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1068,6 +1068,7 @@ struct lock_manager_operations {
>  	int (*lm_change)(struct file_lock *, int, struct list_head *);
>  	void (*lm_setup)(struct file_lock *, void **);
>  	bool (*lm_breaker_owns_lease)(struct file_lock *);
> +	bool (*lm_lock_conflict)(struct file_lock *cfl);
>  };
>  
>  struct lock_manager {

Acked-by: Jeff Layton <jlayton@redhat.com>