Re: [PATCH 2/4] locks: remove conditional lock release in middle of flock_lock_file

[J. Bruce Fields]

On Tue, Feb 17, 2015 at 12:56:49PM -0500, Jeff Layton wrote:
> On Tue, 17 Feb 2015 12:10:17 -0500
> "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> 
> > On Tue, Feb 17, 2015 at 07:46:28AM -0500, Jeff Layton wrote:
> > > As Linus pointed out:
> > > 
> > >     Say we have an existing flock, and now do a new one that conflicts. I
> > >     see what looks like three separate bugs.
> > > 
> > >      - We go through the first loop, find a lock of another type, and
> > >     delete it in preparation for replacing it
> > > 
> > >      - we *drop* the lock context spinlock.
> > > 
> > >      - BUG #1? So now there is no lock at all, and somebody can come in
> > >     and see that unlocked state. Is that really valid?
> > > 
> > >      - another thread comes in while the first thread dropped the lock
> > >     context lock, and wants to add its own lock. It doesn't see the
> > >     deleted or pending locks, so it just adds it
> > > 
> > >      - the first thread gets the context spinlock again, and adds the lock
> > >     that replaced the original
> > > 
> > >      - BUG #2? So now there are *two* locks on the thing, and the next
> > >     time you do an unlock (or when you close the file), it will only
> > >     remove/replace the first one.
> > > 
> > > ...remove the "drop the spinlock" code in the middle of this function as
> > > it has always been suspicious.
> > 
> > Looking back through a historical git repo, purely out of curiosity--the
> > cond_resched was previously a yield, and then I *think* bug #2 was
> > introduced in 2002 by a "[PATCH] fs/locks.c: more cleanup".  Before that
> > it retried the first loop after the yield.
> > 
> > Before that the yield was in locks_wake_up_blocks, removed by:
> > 
> > 	commit 7962ad19e6300531784722c16849837864304d84
> > 	Author: Matthew Wilcox <willy@xxxxxxxxxx>
> > 	Date:   Sat Jun 8 02:09:24 2002 -0700
> > 
> > 	[PATCH] fs/locks.c: Only yield once for flocks
> > 	    
> > 	This patch removes the annoying and confusing `wait' argument
> > 	from many places.  The only change in behaviour is that we now
> > 	yield once when unblocking other BSD-style flocks instead of
> > 	once for each lock.
> > 			    
> > 	This slightly improves the semantics for userspace.  Before,
> > 	when we had two tasks waiting on a lock, the first one would
> > 	receive the lock.  Now, the one with the highest priority
> > 	receives the lock.
> > 
> > So this really was intended to guarantee other waiters the lock before
> > allowing an upgrade.  Could that actually have worked?
> > 
> > The non-atomic behavior is documented in flock(2), which says it's
> > "original BSD behavior".
> > 
> > A comment at the top of locks.c says this is to avoid deadlock.  Hm, so,
> > are we introducing a deadlock?:
> > 
> > 	1. Two processes both get shared lock on different filps.
> > 	2. Both request exclusive lock.
> > 
> > Now they're stuck, whereas previously they might have recovered?
> > 
> > --b.
> > 
> 
> 
> Yes, I see that now. It might be best to preserve the existing behavior
> for that reason then. I'd rather not introduce any behavioral changes in this
> set if we can help it, particularly if there are userland apps that
> might rely on it.
> 
> It may be best then to keep this patch, but drop patch #3.

Unfortunately it's this patch that I'm worried about.

Also these patches are introducing some failure in my locking tests
(probably unrelated, I doubt I ever wrote a test for this case).  I'll
investigate.

--b.

> That should
> be enough to ensure that we don't end up with two different locks on
> the same file description, which is clearly a bug.
> 
> It might also not hurt to follow HCH's earlier advice and make
> locks_remove_flock just iterate over the list and just unconditionally
> delete any lock that in the case where the ->flock operation isn't set.
> 
> In principle we shouldn't need that once apply patch #2, but it would
> probably be simpler than dealing with flock_lock_file in that case.
> 
> > > 
> > > Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
> > > ---
> > >  fs/locks.c | 10 ----------
> > >  1 file changed, 10 deletions(-)
> > > 
> > > diff --git a/fs/locks.c b/fs/locks.c
> > > index 7998f670812c..00c105f499a2 100644
> > > --- a/fs/locks.c
> > > +++ b/fs/locks.c
> > > @@ -901,16 +901,6 @@ static int flock_lock_file(struct file *filp, struct file_lock *request)
> > >  		goto out;
> > >  	}
> > >  
> > > -	/*
> > > -	 * If a higher-priority process was blocked on the old file lock,
> > > -	 * give it the opportunity to lock the file.
> > > -	 */
> > > -	if (found) {
> > > -		spin_unlock(&ctx->flc_lock);
> > > -		cond_resched();
> > > -		spin_lock(&ctx->flc_lock);
> > > -	}
> > > -
> > >  find_conflict:
> > >  	list_for_each_entry(fl, &ctx->flc_flock, fl_list) {
> > >  		if (!flock_locks_conflict(request, fl))
> > > -- 
> > > 2.1.0
> 
> 
> -- 
> Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
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