Re: [PATCH 5/5] fs/locks: create a tree of dependent requests.

[J. Bruce Fields]

On Fri, Aug 10, 2018 at 08:19:26AM +1000, NeilBrown wrote:
> On Thu, Aug 09 2018, J. Bruce Fields wrote:
> > I think you could simplify the code a lot by maintaining the tree so
> > that it always satisfies the condition that waiters are always strictly
> > "weaker" than their descendents, so that finding a conflict with a
> > waiter is always enough to know that the descendents also conflict.
> 
> Can you define "weaker" please.
> I suspect it is a partial ordering, in which case a tree would normally
> be more appropriate than trying to find a total ordering.

Lock X is stronger than lock Y if any lock that would conflict with lock
Y would also conflict with lock X.

Equivalently, X is stronger than Y if lock X's range is a superset of
lock Y's and if X is a write lock whenever Y is.  Well, I *thought* that
was equivalent until I thought about the owner problem.  Ugh.

--b.

> 
> Thanks,
> NeilBrown
> 
> >
> > So, when you put a waiter to sleep, you don't add it below a child
> > unless it's "stronger" than the child.
> >
> > You give up the property that siblings don't conflict, but again that
> > just means thundering herds in weird cases, which is OK.
> >
> > --b.
> >
> >> 
> >> Reported-and-tested-by: Martin Wilck <mwilck@xxxxxxx>
> >> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
> >> ---
> >>  fs/locks.c |   69 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
> >>  1 file changed, 63 insertions(+), 6 deletions(-)
> >> 
> >> diff --git a/fs/locks.c b/fs/locks.c
> >> index fc64016d01ee..17843feb6f5b 100644
> >> --- a/fs/locks.c
> >> +++ b/fs/locks.c
> >> @@ -738,6 +738,39 @@ static void locks_delete_block(struct file_lock *waiter)
> >>  	spin_unlock(&blocked_lock_lock);
> >>  }
> >>  
> >> +static void wake_non_conflicts(struct file_lock *waiter, struct file_lock *blocker,
> >> +			       enum conflict conflict(struct file_lock *,
> >> +						      struct file_lock *))
> >> +{
> >> +	struct file_lock *parent = waiter;
> >> +	struct file_lock *fl;
> >> +	struct file_lock  *t;
> >> +
> >> +	fl = list_entry(&parent->fl_blocked, struct file_lock, fl_block);
> >> +restart:
> >> +	list_for_each_entry_safe_continue(fl, t, &parent->fl_blocked, fl_block) {
> >> +		switch (conflict(fl, blocker)) {
> >> +		default:
> >> +		case FL_NO_CONFLICT:
> >> +			__locks_wake_one(fl);
> >> +			break;
> >> +		case FL_CONFLICT:
> >> +			/* Need to check children */
> >> +			parent = fl;
> >> +			fl = list_entry(&parent->fl_blocked, struct file_lock, fl_block);
> >> +			goto restart;
> >> +		case FL_TRANSITIVE_CONFLICT:
> >> +			/* all children must also conflict, no need to check */
> >> +			continue;
> >> +		}
> >> +	}
> >> +	if (parent != waiter) {
> >> +		parent = parent->fl_blocker;
> >> +		fl = parent;
> >> +		goto restart;
> >> +	}
> >> +}
> >> +
> >>  /* Insert waiter into blocker's block list.
> >>   * We use a circular list so that processes can be easily woken up in
> >>   * the order they blocked. The documentation doesn't require this but
> >> @@ -747,11 +780,32 @@ static void locks_delete_block(struct file_lock *waiter)
> >>   * fl_blocked list itself is protected by the blocked_lock_lock, but by ensuring
> >>   * that the flc_lock is also held on insertions we can avoid taking the
> >>   * blocked_lock_lock in some cases when we see that the fl_blocked list is empty.
> >> + *
> >> + * Rather than just adding to the list, we check for conflicts with any existing
> >> + * waiter, and add to that waiter instead.
> >> + * Thus wakeups don't happen until needed.
> >>   */
> >>  static void __locks_insert_block(struct file_lock *blocker,
> >> -					struct file_lock *waiter)
> >> +				 struct file_lock *waiter,
> >> +				 enum conflict conflict(struct file_lock *,
> >> +							struct file_lock *))
> >>  {
> >> +	struct file_lock *fl;
> >>  	BUG_ON(!list_empty(&waiter->fl_block));
> >> +
> >> +	/* Any request in waiter->fl_blocked is know to conflict with
> >> +	 * waiter, but it might not conflict with blocker.
> >> +	 * If it doesn't, it needs to be woken now so it can find
> >> +	 * somewhere else to wait, or possible it can get granted.
> >> +	 */
> >> +	if (conflict(waiter, blocker) != FL_TRANSITIVE_CONFLICT)
> >> +		wake_non_conflicts(waiter, blocker, conflict);
> >> +new_blocker:
> >> +	list_for_each_entry(fl, &blocker->fl_blocked, fl_block)
> >> +		if (conflict(fl, waiter)) {
> >> +			blocker =  fl;
> >> +			goto new_blocker;
> >> +		}
> >>  	waiter->fl_blocker = blocker;
> >>  	list_add_tail(&waiter->fl_block, &blocker->fl_blocked);
> >>  	if (IS_POSIX(blocker) && !IS_OFDLCK(blocker))
> >> @@ -760,10 +814,12 @@ static void __locks_insert_block(struct file_lock *blocker,
> >>  
> >>  /* Must be called with flc_lock held. */
> >>  static void locks_insert_block(struct file_lock *blocker,
> >> -					struct file_lock *waiter)
> >> +			       struct file_lock *waiter,
> >> +			       enum conflict conflict(struct file_lock *,
> >> +						      struct file_lock *))
> >>  {
> >>  	spin_lock(&blocked_lock_lock);
> >> -	__locks_insert_block(blocker, waiter);
> >> +	__locks_insert_block(blocker, waiter, conflict);
> >>  	spin_unlock(&blocked_lock_lock);
> >>  }
> >>  
> >> @@ -1033,7 +1089,7 @@ static int flock_lock_inode(struct inode *inode, struct file_lock *request)
> >>  		if (!(request->fl_flags & FL_SLEEP))
> >>  			goto out;
> >>  		error = FILE_LOCK_DEFERRED;
> >> -		locks_insert_block(fl, request);
> >> +		locks_insert_block(fl, request, flock_locks_conflict);
> >>  		goto out;
> >>  	}
> >>  	if (request->fl_flags & FL_ACCESS)
> >> @@ -1107,7 +1163,8 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
> >>  			spin_lock(&blocked_lock_lock);
> >>  			if (likely(!posix_locks_deadlock(request, fl))) {
> >>  				error = FILE_LOCK_DEFERRED;
> >> -				__locks_insert_block(fl, request);
> >> +				__locks_insert_block(fl, request,
> >> +						     posix_locks_conflict);
> >>  			}
> >>  			spin_unlock(&blocked_lock_lock);
> >>  			goto out;
> >> @@ -1581,7 +1638,7 @@ int __break_lease(struct inode *inode, unsigned int mode, unsigned int type)
> >>  		break_time -= jiffies;
> >>  	if (break_time == 0)
> >>  		break_time++;
> >> -	locks_insert_block(fl, new_fl);
> >> +	locks_insert_block(fl, new_fl, leases_conflict);
> >>  	trace_break_lease_block(inode, new_fl);
> >>  	spin_unlock(&ctx->flc_lock);
> >>  	percpu_up_read_preempt_enable(&file_rwsem);
> >>