Re: WARNING: at net/ipv4/tcp_input.c:2927 tcp_ack+0xd55/0x1991()

[Jarek Poplawski]

On Mon, Mar 30, 2009 at 08:01:50AM -0600, Jonathan Corbet wrote:
> On Mon, 30 Mar 2009 08:51:07 +0000
> Jarek Poplawski <jarkao2@xxxxxxxxx> wrote:
> 
> > Probably I miss something, but generally in a case like this "a_lock"
> > doesn't have to be taken in IRQ mode to be dangerous. Eg. if one cpu
> > is trying to take this lock after fasync_lock (with IRQs disabled),
> > while another cpu is waiting for fasync_lock in IRQ, which preempted
> > such "a_lock".
> 
> The possibility exists, I guess, yes.
> 
> > Could you give some details of this fix?
> 
> I just reverse the order of lock acquisition in fasync_helper().  Patch
> is attached.  I'll be sending up a pull request shortly.

Yes, this patch should fix this. (And I can see it in the linux-next
now...)

Thanks,
Jarek P.

> 
> jon
> 
> From 4a6a4499693a419a20559c41e33a7bd70bf20a6f Mon Sep 17 00:00:00 2001
> From: Jonathan Corbet <corbet@xxxxxxx>
> Date: Fri, 27 Mar 2009 12:24:31 -0600
> Subject: [PATCH] Fix a lockdep warning in fasync_helper()
> 
> Lockdep gripes if file->f_lock is taken in a no-IRQ situation, since that
> is not always the case.  We don't really want to disable IRQs for every
> acquisition of f_lock; instead, just move it outside of fasync_lock.
> 
> Reported-by: Bartlomiej Zolnierkiewicz <bzolnier@xxxxxxxxx>
> Reported-by: Larry Finger <Larry.Finger@xxxxxxxxxxxx>
> Reported-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> Signed-off-by: Jonathan Corbet <corbet@xxxxxxx>
> ---
>  fs/fcntl.c         |   10 +++++++---
>  include/linux/fs.h |    2 +-
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index d865ca6..cc8e4de 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -531,6 +531,12 @@ int fasync_helper(int fd, struct file * filp, int on, struct fasync_struct **fap
>  		if (!new)
>  			return -ENOMEM;
>  	}
> +
> +	/*
> +	 * We need to take f_lock first since it's not an IRQ-safe
> +	 * lock.
> +	 */
> +	spin_lock(&filp->f_lock);
>  	write_lock_irq(&fasync_lock);
>  	for (fp = fapp; (fa = *fp) != NULL; fp = &fa->fa_next) {
>  		if (fa->fa_file == filp) {
> @@ -555,14 +561,12 @@ 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;
>  }
>  
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 7428c6d..2f13c1d 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -848,7 +848,7 @@ struct file {
>  #define f_dentry	f_path.dentry
>  #define f_vfsmnt	f_path.mnt
>  	const struct file_operations	*f_op;
> -	spinlock_t		f_lock;  /* f_ep_links, f_flags */
> +	spinlock_t		f_lock;  /* f_ep_links, f_flags, no IRQ */
>  	atomic_long_t		f_count;
>  	unsigned int 		f_flags;
>  	fmode_t			f_mode;
> -- 
> 1.6.2
> 
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