RE: NFS root lockups with -next 20110113

[Santosh Shilimkar <santosh.shilimkar@ti.com>]

> -----Original Message-----
> From: J. R. Okajima [mailto:hooanon05@yahoo.co.jp]
> Sent: Thursday, January 13, 2011 6:52 PM
> To: Mark Brown
> Cc: Trond Myklebust; Santosh Shilimkar; Nick Piggin; linux-
> nfs@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
> fsdevel@vger.kernel.org
> Subject: Re: NFS root lockups with -next 20110113
>
>
> Mark Brown:
> > With -next 20110113 I'm experiencing lockups shortly after
> userspace
> > starts when booting with my root filesystem on NFS, log below.  I
> can
> > boot into /bin/sh but running real userspace triggers this very
> easily.
> > This was introduced sometime this week.
> >
> > I've not bisected or otherwise investigated much yet, but I do
> notice
> > code added recently by Nick in "fs: rcu-walk for path lookup"
> showing up
> > in the backtrace so including him in the CCs.
>
> This and a report from Santosh Shilimkar look like the same problem
> which I reported, and I am testing this patch. If you can, please
> try it
> too.
> Note: Of course this is not offcial fix.
>

It works. My board booted with NFS with below patch


>
> diff --git a/fs/namei.c b/fs/namei.c
> index 5bb7588..51d052f 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -480,6 +480,7 @@ static int nameidata_dentry_drop_rcu(struct
> nameidata *nd, struct dentry *dentry
>  {
>  	struct fs_struct *fs = current->fs;
>  	struct dentry *parent = nd->path.dentry;
> +	int isroot;
>
>  	BUG_ON(!(nd->flags & LOOKUP_RCU));
>  	if (nd->root.mnt) {
> @@ -489,18 +490,22 @@ static int nameidata_dentry_drop_rcu(struct
> nameidata *nd, struct dentry *dentry
>  			goto err_root;
>  	}
>  	spin_lock(&parent->d_lock);
> -	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
> -	if (!__d_rcu_to_refcount(dentry, nd->seq))
> -		goto err;
> +	isroot = IS_ROOT(dentry);
> +	if (!isroot) {
> +		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
> +		if (!__d_rcu_to_refcount(dentry, nd->seq))
> +			goto err;
> +	}
>  	/*
>  	 * If the sequence check on the child dentry passed, then the
> child has
>  	 * not been removed from its parent. This means the parent
> dentry must
>  	 * be valid and able to take a reference at this point.
>  	 */
> -	BUG_ON(!IS_ROOT(dentry) && dentry->d_parent != parent);
> +	BUG_ON(!isroot && dentry->d_parent != parent);
>  	BUG_ON(!parent->d_count);
>  	parent->d_count++;
> -	spin_unlock(&dentry->d_lock);
> +	if (!isroot)
> +		spin_unlock(&dentry->d_lock);
>  	spin_unlock(&parent->d_lock);
>  	if (nd->root.mnt) {
>  		path_get(&nd->root);
> @@ -513,7 +518,8 @@ static int nameidata_dentry_drop_rcu(struct
> nameidata *nd, struct dentry *dentry
>  	nd->flags &= ~LOOKUP_RCU;
>  	return 0;
>  err:
> -	spin_unlock(&dentry->d_lock);
> +	if (!isroot)
> +		spin_unlock(&dentry->d_lock);
>  	spin_unlock(&parent->d_lock);
>  err_root:
>  	if (nd->root.mnt)