Re: fs/locks.c BKL removal

[Matthew Wilcox <willy@debian.org>]

On Fri, May 10, 2002 at 02:48:39PM -0700, Dave Hansen wrote:
> I'm looking into the fs/locks.c mess.  It appears that there was an 
> attempt to convert this over to a semaphore, but it was removed just 
> before the 2.4 release because of some deadlocks.

Actually, performance problems ...

> Whenever the i_flock list is traversed, the BKL is held.  It is also 
> held while running through the file_lock_list which I think is used 
> only for /proc/locks.

Correct.

> We definitely need a semaphore because of all the blocking that goes 
> on.  We can either have a global lock for all of them, which I think 
> was tried last time.  Or, we can split it up a bit more.  With the 
> current design, there will need to be a lock for the global list, each 
> individual list, and one for each individual lock to protect against 
> access from the reference in the file_lock_list and the inode->i_flock 
> list.

Nah.  Though I'm glad you missed it too; it means that I'm not as
stupid as I thought I was for only noticing it 2 years later.  Look at
locks_wake_up_blocks (this is basically the _only_ tricky part).  This has
to be called with a wait argument which is true.  The only time that
can happen is if locks_delete_lock is called with a `true' parameter.
And the only time _that_ happens is when the _type_ of an flock lock is
being changed.

And really, what's happening here?  We have a BSD flock which is blocking
one or more locks.  Those processes have to have the opportunity
to acquire the lock before the previously-blocking process gets the
opportunity to acquire its lock.  But that doesn't mean we need to schedule once
for _each_ task which is blocked; we only need to yield once.

So we can eliminate the `wait' argument to locks_delete_lock,
locks_wake_up_blocks and the arm of the conditional in
locks_wake_up_blocks which sleeps.  We only need to check in
flock_lock_file whether we're unlocking and yield if we aren't.

I'm currently doing a major restructure of fs/locks.c, and this problem
(along with several others) simply disappears.  I'm looking for a
testsuite before I release this code to the world ... anybody got one?

> However, I think that the file_lock_list complexity may be able to be 
> reduced.  If we make the file_lock_list a list of inodes (or just the 
> i_flocks) with active locks, we can avoid the complexity of having an 
> individual file_lock lock.  That way, we at least reduce the number of 
> _types_ of locks.  It increases the number of dereferences, but this 
> is /proc we're talking about.  Any comments?

Ick... I'd really like to see one spinlock protecting all activity in this
area.  And obviously not the magic BKL ;-)

> Talking about locks for locks is confusing :)

Tell me about it!  I'm close to calling things `blocks' `plocks',
`leases' and `mlocks', just to reduce the namespace conflicts.  But it's
not obvious those refer to BSD locks, POSIX locks and Mandatory locks...

-- 
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