Re: [PATCH v3 5/6] locks: report l_pid as -1 for FL_FILE_PVT locks

[Jeff Layton]

On Tue, 10 Dec 2013 14:17:34 -0500
Jeff Layton <jlayton@xxxxxxxxxx> wrote:

> FL_FILE_PVT locks are no longer tied to a particular pid, and are
> instead inheritable by child processes. Report a l_pid of '-1' for
> these sorts of locks since the pid is somewhat meaningless for them.
> 
> This precedent comes from FreeBSD. There, POSIX and flock() locks can
> conflict with one another. If fcntl(F_GETLK, ...) returns a lock set
> with flock() then the l_pid member cannot be a process ID because the
> lock is not held by a process as such.
> 
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
>  fs/locks.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index e163a30..5372ddd 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -1899,7 +1899,7 @@ EXPORT_SYMBOL_GPL(vfs_test_lock);
>  
>  static int posix_lock_to_flock(struct flock *flock, struct file_lock *fl)
>  {
> -	flock->l_pid = fl->fl_pid;
> +	flock->l_pid = IS_FILE_PVT(fl) ? -1 : fl->fl_pid;
>  #if BITS_PER_LONG == 32
>  	/*
>  	 * Make sure we can represent the posix lock via
> @@ -1921,7 +1921,7 @@ static int posix_lock_to_flock(struct flock *flock, struct file_lock *fl)
>  #if BITS_PER_LONG == 32
>  static void posix_lock_to_flock64(struct flock64 *flock, struct file_lock *fl)
>  {
> -	flock->l_pid = fl->fl_pid;
> +	flock->l_pid = IS_FILE_PVT(fl) ? -1 : fl->fl_pid;
>  	flock->l_start = fl->fl_start;
>  	flock->l_len = fl->fl_end == OFFSET_MAX ? 0 :
>  		fl->fl_end - fl->fl_start + 1;

While I think this behavior is reasonable, I do wonder if we ought to
consider more changes to how F_GETLK works. Currently the F_GETLK code
won't handle the new l_type values, but maybe it should...

For instance, if there is a conflicting lock, and the F_GETLK caller
specified F_RDLCKP or F_WRLCKP, might it make sense to report the
l_type on the conflicting lock as F_RDLCKP or F_WRLCKP if that
conflicting lock is also a *P type?

...or maybe we should consider a new F_GETLKP cmd value, and a new
expanded struct flock that gives more info. The pid is already somewhat
meaningless with this sort of lock. Perhaps we could obfuscate the
fl_owner value and report that instead? What other sorts of info would
be useful to programs that intend to use these new interfaces?

-- 
Jeff Layton <jlayton@xxxxxxxxxx>
--
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