linux-kernel - Re: [PATCH] fs/locks.c: prepare for BKL removal

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Date:	Sun, 19 Sep 2010 15:34:18 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-kernel@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>,
	Christoph Hellwig <hch@....de>,
	Trond Myklebust <trond.myklebust@....uio.no>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, John Kacur <jkacur@...hat.com>,
	Sage Weil <sage@...dream.net>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs/locks.c: prepare for BKL removal

On Sat, Sep 18, 2010 at 03:09:31PM +0200, Arnd Bergmann wrote:
> This prepares the removal of the big kernel lock from the
> file locking code. We still use the BKL as long as fs/lockd
> uses it and ceph might sleep, but we can flip the definition
> to a private spinlock as soon as that's done.
> All users outside of fs/lockd get converted to use
> lock_flocks() instead of lock_kernel() where appropriate.
> 
> Based on an earlier patch to use a spinlock from Matthew
> Wilcox, who has attempted this a few times before. An even
> earlier attempt to use a semaphore instead of the BKL
> apparently was made by Andrew Morton about ten years ago,
> but reverted for performance reasons.
> 
> Someone should do some serious performance testing when
> this becomes a spinlock, since this has caused problems
> before. Using a spinlock should be at least as good
> as the BKL in theory, but who knows...
> 
> If nobody has any objections to this preparation patch,
> I'd like to add it to my bkl/vfs tree and into -next.

Looks good to me.

>  EXPORT_SYMBOL(posix_test_lock);
> @@ -730,18 +746,16 @@ static int flock_lock_file(struct file *filp, struct file_lock *request)
>  	int error = 0;
>  	int found = 0;
>  
> -	lock_kernel();
> -	if (request->fl_flags & FL_ACCESS)
> -		goto find_conflict;
> -
> -	if (request->fl_type != F_UNLCK) {
> -		error = -ENOMEM;
> +	if (!(request->fl_flags & FL_ACCESS) && (request->fl_type != F_UNLCK)) {
>  		new_fl = locks_alloc_lock();
> -		if (new_fl == NULL)
> -			goto out;
> -		error = 0;
> +		if (!new_fl)
> +			return -ENOMEM;
>  	}
>  
> +	lock_flocks();
> +	if (request->fl_flags & FL_ACCESS)
> +		goto find_conflict;
> +

I might have left this to a separate patch, but OK.

--b.

>  	for_each_lock(inode, before) {
>  		struct file_lock *fl = *before;
>  		if (IS_POSIX(fl))
> @@ -767,8 +781,11 @@ static int flock_lock_file(struct file *filp, struct file_lock *request)
>  	 * If a higher-priority process was blocked on the old file lock,
>  	 * give it the opportunity to lock the file.
>  	 */
> -	if (found)
> +	if (found) {
> +		unlock_flocks();
>  		cond_resched();
> +		lock_flocks();
> +	}
>  
>  find_conflict:
>  	for_each_lock(inode, before) {
--
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