linux-kernel - Re: [RFC PATCH v2 3/5] locks: add new "private" lock type that is owned by the filp

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Date:	Wed, 20 Nov 2013 14:50:32 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	nfs-ganesha-devel@...ts.sourceforge.net,
	samba-technical@...ts.samba.org
Subject: Re: [RFC PATCH v2 3/5] locks: add new "private" lock type that is
 owned by the filp

On Wed, Nov 20, 2013 at 11:45:04AM -0500, Jeff Layton wrote:
> Due to some unfortunate history, POSIX locks have very strange and
> unhelpful semantics. The thing that usually catches people by surprise
> is that they are dropped whenever the process closes any file descriptor
> associated with the inode.
> 
> This is extremely problematic for people developing file servers that
> need to implement byte-range locks. Developers often need a "lock
> management" facility to ensure that file descriptors are not closed
> until all of the locks associated with the inode are finished.
> 
> This patchset adds a new type of lock that attempts to address this
> issue. These locks work just like "normal" POSIX read/write locks, but
> have semantics that are more like BSD locks with respect to inheritance
> and behavior on close.
> 
> This is implemented primarily by changing how fl_owner field is set for
> these locks. Instead of having them owned by the files_struct of the
> process, they are instead owned by the filp on which they were acquired.
> Thus, they are inherited across fork() and are only released when the
> last reference to a filp is put.
> 
> These new semantics prevent them from being merged with "classic" POSIX
> locks, even if they are acquired by the same process. These locks will
> also conflict with "classic" POSIX locks even if they are acquired by
> the same process or on the same file descriptor.
> 
> Signed-off-by: Jeff Layton <jlayton@...hat.com>
> ---
>  fs/locks.c                       | 22 +++++++++++++++++++++-
>  include/uapi/asm-generic/fcntl.h | 15 +++++++++++++++
>  2 files changed, 36 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index 86cafc3..3b278a6 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -348,6 +348,26 @@ static int posix_assign_type(struct file_lock *fl, long type)
>  {
>  	int err;
>  
> +	/*
> +	 * FL_FILP_PRIVATE locks are "owned" by the filp upon which they were
> +	 * acquired, regardless of what task is dealing with them. Set the
> +	 * fl_owner appropriately.
> +	 */
> +	switch (type) {
> +	case F_RDLCKP:
> +		type = F_RDLCK;
> +		fl->fl_owner = (fl_owner_t)fl->fl_file;
> +		break;
> +	case F_WRLCKP:
> +		type = F_WRLCK;
> +		fl->fl_owner = (fl_owner_t)fl->fl_file;
> +		break;
> +	case F_UNLCKP:
> +		type = F_UNLCK;
> +		fl->fl_owner = (fl_owner_t)fl->fl_file;
> +		break;
> +	}
> +

After this fl_owner gets set to current->files in
flock{64}_to_posix_lock and then reset here.  That seems like a trap for
the unwary reader.

Could you do something like rename this flock_to_posix_lock_common and
move all the 32/64-bit-independent initialization here?

Looks like there's way more duplication than necessary between those two
cases.

(Also, why do we have an fl_owner_t instead of using a void?)

--b.

>  	err = assign_type(fl, type);
>  	if (err)
>  		return err;
> @@ -2225,7 +2245,7 @@ void locks_remove_filp(struct file *filp)
>  
>  	while ((fl = *before) != NULL) {
>  		if (fl->fl_file == filp) {
> -			if (IS_FLOCK(fl)) {
> +			if (IS_FLOCK(fl) || IS_POSIX(fl)) {
>  				locks_delete_lock(before);
>  				continue;
>  			}
> diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> index 95e46c8..6b7b68a 100644
> --- a/include/uapi/asm-generic/fcntl.h
> +++ b/include/uapi/asm-generic/fcntl.h
> @@ -151,6 +151,21 @@ struct f_owner_ex {
>  #define F_UNLCK		2
>  #endif
>  
> +/*
> + * fd "private" POSIX locks.
> + *
> + * Usually POSIX locks held by a process are released on *any* close and are
> + * not inherited across a fork().
> + *
> + * These lock types will conflict with normal POSIX locks, but are "owned"
> + * by the fd, not the process. This means that they are inherited across
> + * fork() like BSD (flock) locks, and they are only closed when the last
> + * reference to the the filp against which were acquired is closed.
> + */
> +#define F_RDLCKP	5
> +#define F_WRLCKP	6
> +#define F_UNLCKP	7
> +
>  /* for old implementation of bsd flock () */
>  #ifndef F_EXLCK
>  #define F_EXLCK		4	/* or 3 */
> -- 
> 1.8.3.1
> 
> --
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