Re: [PATCH RFC v23 5/7] fs/lock: add 2 callbacks to lock_manager_operations to resolve conflict

["J. Bruce Fields" <bfields@fieldses.org>]

On Thu, Apr 28, 2022 at 12:06:33AM -0700, Dai Ngo wrote:
> Add 2 new callbacks, lm_lock_expirable and lm_expire_lock, to
> lock_manager_operations to allow the lock manager to take appropriate
> action to resolve the lock conflict if possible.
> 
> A new field, lm_mod_owner, is also added to lock_manager_operations.
> The lm_mod_owner is used by the fs/lock code to make sure the lock
> manager module such as nfsd, is not freed while lock conflict is being
> resolved.
> 
> lm_lock_expirable checks and returns true to indicate that the lock
> conflict can be resolved else return false. This callback must be
> called with the flc_lock held so it can not block.
> 
> lm_expire_lock is called to resolve the lock conflict if the returned
> value from lm_lock_expirable is true. This callback is called without
> the flc_lock held since it's allowed to block. Upon returning from
> this callback, the lock conflict should be resolved and the caller is
> expected to restart the conflict check from the beginnning of the list.
> 
> 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 |  4 ++++
>  fs/locks.c                            | 45 +++++++++++++++++++++++++++++++----
>  include/linux/fs.h                    |  3 +++
>  3 files changed, 48 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
> index c26d854275a0..0997a258361a 100644
> --- a/Documentation/filesystems/locking.rst
> +++ b/Documentation/filesystems/locking.rst
> @@ -428,6 +428,8 @@ 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_expirable)(struct file_lock *);
> +        void (*lm_expire_lock)(void);
>  
>  locking rules:
>  
> @@ -439,6 +441,8 @@ lm_grant:		no		no			no
>  lm_break:		yes		no			no
>  lm_change		yes		no			no
>  lm_breaker_owns_lease:	yes     	no			no
> +lm_lock_expirable	yes		no			no
> +lm_expire_lock		no		no			yes
>  ======================	=============	=================	=========
>  
>  buffer_head
> diff --git a/fs/locks.c b/fs/locks.c
> index c369841ef7d1..d48c3f455657 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -896,6 +896,37 @@ static bool flock_locks_conflict(struct file_lock *caller_fl,
>  	return locks_conflict(caller_fl, sys_fl);
>  }
>  
> +static bool
> +resolve_lock_conflict_locked(struct file_lock_context *ctx,
> +			struct file_lock *cfl, bool rwsem)
> +{
> +	void *owner;
> +	bool ret;
> +	void (*func)(void);
> +
> +	if (cfl->fl_lmops && cfl->fl_lmops->lm_lock_expirable &&
> +				cfl->fl_lmops->lm_expire_lock) {
> +		ret = (*cfl->fl_lmops->lm_lock_expirable)(cfl);
> +		if (!ret)
> +			return false;
> +		owner = cfl->fl_lmops->lm_mod_owner;
> +		if (!owner)
> +			return false;
> +		func = cfl->fl_lmops->lm_expire_lock;
> +		__module_get(owner);
> +		if (rwsem)
> +			percpu_up_read(&file_rwsem);
> +		spin_unlock(&ctx->flc_lock);

Dropping and reacquiring locks inside a function like this makes me
nervous.  It means it's not obvious in the caller that the lock isn't
held throughout.

I know it's more verbose, but let's just open-code this logic in the
callers.

(And, thanks for catching the test_lock case, I'd forgotten it.)

Also: do we *really* need to drop the file_rwsem?  Were you seeing it
that cause problems?  The only possible conflict is with someone trying
to read /proc/locks, and I'm surprised that it'd be a problem to let
them wait here.

--b.

> +		(*func)();
> +		module_put(owner);
> +		if (rwsem)
> +			percpu_down_read(&file_rwsem);
> +		spin_lock(&ctx->flc_lock);
> +		return true;
> +	}
> +	return false;
> +}
> +
>  void
>  posix_test_lock(struct file *filp, struct file_lock *fl)
>  {
> @@ -910,11 +941,14 @@ posix_test_lock(struct file *filp, struct file_lock *fl)
>  	}
>  
>  	spin_lock(&ctx->flc_lock);
> +retry:
>  	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 (resolve_lock_conflict_locked(ctx, cfl, false))
> +			goto retry;
> +		locks_copy_conflock(fl, cfl);
> +		goto out;
>  	}
>  	fl->fl_type = F_UNLCK;
>  out:
> @@ -1108,6 +1142,7 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
>  
>  	percpu_down_read(&file_rwsem);
>  	spin_lock(&ctx->flc_lock);
> +retry:
>  	/*
>  	 * New lock request. Walk all POSIX locks and look for conflicts. If
>  	 * there are any, either return error or put the request on the
> @@ -1117,6 +1152,8 @@ 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 (resolve_lock_conflict_locked(ctx, fl, true))
> +				goto retry;
>  			if (conflock)
>  				locks_copy_conflock(conflock, fl);
>  			error = -EAGAIN;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index b8ed7f974fb4..aa6c1bbdb8c4 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1029,6 +1029,7 @@ struct file_lock_operations {
>  };
>  
>  struct lock_manager_operations {
> +	void *lm_mod_owner;
>  	fl_owner_t (*lm_get_owner)(fl_owner_t);
>  	void (*lm_put_owner)(fl_owner_t);
>  	void (*lm_notify)(struct file_lock *);	/* unblock callback */
> @@ -1037,6 +1038,8 @@ 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_expirable)(struct file_lock *cfl);
> +	void (*lm_expire_lock)(void);
>  };
>  
>  struct lock_manager {
> -- 
> 2.9.5