linux-kernel - Re: [GIT PULL] please pull file-locking related changes for v3.20

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Date:	Tue, 17 Feb 2015 11:41:40 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jeff Layton <jlayton@...chiereds.net>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [GIT PULL] please pull file-locking related changes for v3.20

On Tue, Feb 17, 2015 at 11:27 AM, Jeff Layton <jlayton@...chiereds.net> wrote:
>
> What about this instead then?

No. Really.

> - leave the "drop the spinlock" thing in place in flock_lock_file for
>   v3.20

No. The whole concept of "drop the lock in the middle" is *BROKEN*.
It's seriously crap. It's not just a bug, it's a really fundamentally
wrong thing to do.

> - change locks_remove_flock to just walk the list and delete any locks
>   associated with the filp being closed

No. That's still wrong. You can have two people holding a write-lock.
Seriously. That's *shit*.

The "drop the spinlock in the middle" must go. There's not even any
reason for it. Just get rid of it. There can be no deadlock if you get
rid of it, because

 - we hold the flc_lock over the whole event, so we can never see any
half-way state

 - if we actually decide to sleep (due to conflicting locks) and
return FILE_LOCK_DEFERRED, we will drop the lock before actually
sleeping, so nobody else will be deadlocking on this file lock. So any
*other* person who tries to do an upgrade will not sleep, because the
pending upgrade will have moved to the blocking list (that whole
"locks_insert_block" part.

Ergo, either we'll upgrade the lock (atomically, within flc_lock), or
we will drop the lock (possibly moving it to the blocking list). I
don't see a deadlock.

I think your (and mine - but mine had the more fundamental problem of
never setting "old_fl" correctly at all) patch had a deadlock because
you didn't actually remove the old lock when you returned
FILE_LOCK_DEFERRED.

But I think the correct minimal patch is actually to just remove the
"if (found)" statement.

                       Linus
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