linux-kernel - Re: [PATCH 2/4] locks: remove conditional lock release in middle of flock_lock

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Date:	Tue, 17 Feb 2015 12:10:17 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jeff Layton <jeff.layton@...marydata.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH 2/4] locks: remove conditional lock release in middle of
 flock_lock_file

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@...ian.org>
	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.

> 
> Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Jeff Layton <jeff.layton@...marydata.com>
> ---
>  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
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