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Date:   Thu, 9 Aug 2018 09:09:59 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     NeilBrown <neilb@suse.com>
Cc:     Jeff Layton <jlayton@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Martin Wilck <mwilck@suse.de>, linux-fsdevel@vger.kernel.org,
        Frank Filz <ffilzlnx@mindspring.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/5] fs/locks: change all *_conflict() functions to
 return a new enum.
Message-ID: <20180809130959.GH23873@fieldses.org>
References: <153378012255.1220.6754153662007899557.stgit@noble>
 <153378028114.1220.3708291796442871726.stgit@noble>
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On Thu, Aug 09, 2018 at 12:04:41PM +1000, NeilBrown wrote:
> In a future patch we will need to differentiate between conflicts that
> are "transitive" and those that aren't.
> A "transitive" conflict is defined as one where any lock that
> conflicts with the first (newly requested) lock would conflict with
> the existing lock.
> 
> So change posix_locks_conflict(), flock_locks_conflict() (both
> currently returning int) and leases_conflict() (currently returning
> bool) to return "enum conflict".
> Add locks_transitive_overlap() to make it possible to compute the
> correct conflict for posix locks.
> 
> The FL_NO_CONFLICT value is zero, so current code which only tests the
> truth value of these functions will still work the same way.
> 
> And convert some
>    return (foo);
> to
>    return foo;
> 
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
>  fs/locks.c |   64 ++++++++++++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 49 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index b4812da2a374..d06658b2dc7a 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -139,6 +139,16 @@
>  #define IS_OFDLCK(fl)	(fl->fl_flags & FL_OFDLCK)
>  #define IS_REMOTELCK(fl)	(fl->fl_pid <= 0)
>  
> +/* A transitive conflict is one where the first lock conflicts with
> + * the second lock, and any other lock that conflicts with the
> + * first lock, also conflicts with the second lock.
> + */
> +enum conflict {
> +	FL_NO_CONFLICT = 0,
> +	FL_CONFLICT,
> +	FL_TRANSITIVE_CONFLICT,
> +};
> +
>  static inline bool is_remote_lock(struct file *filp)
>  {
>  	return likely(!(filp->f_path.dentry->d_sb->s_flags & SB_NOREMOTELOCK));
> @@ -612,6 +622,15 @@ static inline int locks_overlap(struct file_lock *fl1, struct file_lock *fl2)
>  		(fl2->fl_end >= fl1->fl_start));
>  }
>  
> +/* Check for transitive-overlap - true if any lock that overlaps
> + * the first lock must overlap the seconds
> + */
> +static inline bool locks_transitive_overlap(struct file_lock *fl1,
> +					    struct file_lock *fl2)
> +{
> +	return (fl1->fl_start >= fl2->fl_start) &&
> +		(fl1->fl_end <= fl2->fl_end);
> +}
>  /*
>   * Check whether two locks have the same owner.
>   */
> @@ -793,47 +812,61 @@ locks_delete_lock_ctx(struct file_lock *fl, struct list_head *dispose)
>  /* Determine if lock sys_fl blocks lock caller_fl. Common functionality
>   * checks for shared/exclusive status of overlapping locks.
>   */
> -static int locks_conflict(struct file_lock *caller_fl, struct file_lock *sys_fl)
> +static enum conflict locks_conflict(struct file_lock *caller_fl,
> +				    struct file_lock *sys_fl)
>  {
>  	if (sys_fl->fl_type == F_WRLCK)
> -		return 1;
> +		return FL_TRANSITIVE_CONFLICT;
>  	if (caller_fl->fl_type == F_WRLCK)
> -		return 1;
> -	return 0;
> +		return FL_CONFLICT;
> +	return FL_NO_CONFLICT;
>  }
>  
>  /* Determine if lock sys_fl blocks lock caller_fl. POSIX specific
>   * checking before calling the locks_conflict().
>   */
> -static int posix_locks_conflict(struct file_lock *caller_fl, struct file_lock *sys_fl)
> +static enum conflict posix_locks_conflict(struct file_lock *caller_fl,
> +					  struct file_lock *sys_fl)
>  {
>  	/* POSIX locks owned by the same process do not conflict with
>  	 * each other.
>  	 */
>  	if (posix_same_owner(caller_fl, sys_fl))
> -		return (0);
> +		return FL_NO_CONFLICT;
>  
>  	/* Check whether they overlap */
>  	if (!locks_overlap(caller_fl, sys_fl))
> -		return 0;
> +		return FL_NO_CONFLICT;
>  
> -	return (locks_conflict(caller_fl, sys_fl));
> +	switch (locks_conflict(caller_fl, sys_fl)) {
> +	default:
> +	case FL_NO_CONFLICT:
> +		return FL_NO_CONFLICT;
> +	case FL_CONFLICT:
> +		return FL_CONFLICT;

If I'm understanding the logic here and in locks_conflict correctly,
you're telling me that in the case where sys_fl is a read lock, and
caller_fl is a write lock, then any lock which conflicts with sys_fl
must conflict with caller_fl?  Or do I have that backwards?  It doesn't
sound right, in any case.

--b.

> +	case FL_TRANSITIVE_CONFLICT:
> +		if (locks_transitive_overlap(caller_fl, sys_fl))
> +			return FL_TRANSITIVE_CONFLICT;
> +		else
> +			return FL_CONFLICT;
> +	}
>  }
>  
>  /* Determine if lock sys_fl blocks lock caller_fl. FLOCK specific
>   * checking before calling the locks_conflict().
>   */
> -static int flock_locks_conflict(struct file_lock *caller_fl, struct file_lock *sys_fl)
> +static enum conflict flock_locks_conflict(struct file_lock *caller_fl,
> +					  struct file_lock *sys_fl)
>  {
>  	/* FLOCK locks referring to the same filp do not conflict with
>  	 * each other.
>  	 */
>  	if (caller_fl->fl_file == sys_fl->fl_file)
> -		return (0);
> +		return FL_NO_CONFLICT;
>  	if ((caller_fl->fl_type & LOCK_MAND) || (sys_fl->fl_type & LOCK_MAND))
> -		return 0;
> +		return FL_NO_CONFLICT;
>  
> -	return (locks_conflict(caller_fl, sys_fl));
> +	return locks_conflict(caller_fl, sys_fl);
>  }
>  
>  void
> @@ -1435,12 +1468,13 @@ static void time_out_leases(struct inode *inode, struct list_head *dispose)
>  	}
>  }
>  
> -static bool leases_conflict(struct file_lock *lease, struct file_lock *breaker)
> +static enum conflict leases_conflict(struct file_lock *lease,
> +				     struct file_lock *breaker)
>  {
>  	if ((breaker->fl_flags & FL_LAYOUT) != (lease->fl_flags & FL_LAYOUT))
> -		return false;
> +		return FL_NO_CONFLICT;
>  	if ((breaker->fl_flags & FL_DELEG) && (lease->fl_flags & FL_LEASE))
> -		return false;
> +		return FL_NO_CONFLICT;
>  	return locks_conflict(breaker, lease);
>  }
>  
>