Date:   Fri, 10 Aug 2018 09:40:35 +1000
From:   NeilBrown <neilb@...e.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Martin Wilck <mwilck@...e.de>, linux-fsdevel@...r.kernel.org,
        Frank Filz <ffilzlnx@...dspring.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] fs/locks: change all *_conflict() functions to return a new enum.

On Thu, Aug 09 2018, J. Bruce Fields wrote:

> 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@...e.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.

As I was writing this code, I was thinking that I'd probably end up
getting something backwards....
Let's see.  I wrote:

>> +/* 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.
>> + */

caller_fl is first and sys_fl is second.

if sys_fl, the second, is a read lock, and caller_fl, the first, is a
write lock, they clearly conflict but any other lock that conflict
with caller_fl (The write lock) would *not* necessarily conflict with
the read lock.  So this situation is *not*  FL_TRANSITIVE_CONFLICT.

locks_conflict() only returns FL_TRANSITIVE_CONFLICT when sys_fl (the
second) is a write lock, which it isn't in this case.  So I think that
this case is handled correctly.
posix_locks_conflict() will return FL_CONFLICT, but not
FL_TRANSITIVE_CONFLICT.

Have I convinced you, or have I missed your point?

Thanks,
NeilBrown


>
> --b.
>
>> +	case FL_TRANSITIVE_CONFLICT:
>> +		if (locks_transitive_overlap(caller_fl, sys_fl))
>> +			return FL_TRANSITIVE_CONFLICT;
>> +		else
>> +			return FL_CONFLICT;
>> +	}
>>  }

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