linux-kernel - Re: Possible IRQ lock inversion from 2.6.29-Linus-03321-gbe0ea69 (2.6.29-git)

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Date:	Sat, 28 Mar 2009 01:32:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jonathan Corbet <corbet@....net>
Cc:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>,
	Larry Finger <Larry.Finger@...inger.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Li Zefan <lizf@...fujitsu.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Possible IRQ lock inversion from 2.6.29-Linus-03321-gbe0ea69
 (2.6.29-git)

On Fri, 2009-03-27 at 12:06 -0600, Jonathan Corbet wrote:
> On Fri, 27 Mar 2009 13:54:35 +0100
> Bartlomiej Zolnierkiewicz <bzolnier@...il.com> wrote:
> 
> > I remember looking a bit more closely into the issue and not seeing
> > the problem with the locking (though I could have missed something):
> > 
> > file->f_lock is never taken in hard-irq or soft-irq context and in
> > the only place where file->f_lock is taken with fasync_lock hold we're
> > protected against IRQs by write_lock_irq().
> 
> I do think that the warning is spurious at this time.

I think you're right (although at 1:30 am I can't be sure).

It does point to inconsistent (sloppy) lock usage though, because f_lock
is used both with and without irqs disabled -- so on that ground its
correct to complain.

> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index d865ca6..b9c1a4b 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -531,6 +531,7 @@ int fasync_helper(int fd, struct file * filp, int on, struct fasync_struct **fap
>  		if (!new)
>  			return -ENOMEM;
>  	}
> +	spin_lock(&filp->f_lock); /* outside fasync_lock to keep lockdep happy */

Please don't put in comments like that, they're worse than useless.
Either explain in detail how and why, or don't bother.

>  	write_lock_irq(&fasync_lock);
>  	for (fp = fapp; (fa = *fp) != NULL; fp = &fa->fa_next) {
>  		if (fa->fa_file == filp) {
> @@ -555,14 +556,12 @@ int fasync_helper(int fd, struct file * filp, int on, struct fasync_struct **fap
>  		result = 1;
>  	}
>  out:
> -	/* Fix up FASYNC bit while still holding fasync_lock */
> -	spin_lock(&filp->f_lock);
>  	if (on)
>  		filp->f_flags |= FASYNC;
>  	else
>  		filp->f_flags &= ~FASYNC;
> -	spin_unlock(&filp->f_lock);
>  	write_unlock_irq(&fasync_lock);
> +	spin_unlock(&filp->f_lock);
>  	return result;
>  }

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