linux-kernel - Re: [patch 24/52] fs: dcache reduce d

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Message-ID: <20100628215013.GG2357@linux.vnet.ibm.com>
Date:	Mon, 28 Jun 2010 14:50:13 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	npiggin@...e.de
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>,
	Frank Mayhar <fmayhar@...gle.com>
Subject: Re: [patch 24/52] fs: dcache reduce d_parent locking

On Thu, Jun 24, 2010 at 01:02:36PM +1000, npiggin@...e.de wrote:
> Use RCU property of dcache to simplify locking in some places where we
> take d_parent and d_lock.
> 
> Comment: don't need rcu_deref because we take the spinlock and recheck it.

Looks good other than one question below.

							Thanx, Paul

> Signed-off-by: Nick Piggin <npiggin@...e.de>
> --
> 
> Index: linux-2.6/fs/dcache.c
> ===================================================================
> --- linux-2.6.orig/fs/dcache.c
> +++ linux-2.6/fs/dcache.c
> @@ -311,23 +311,18 @@ struct dentry *dget_parent(struct dentry
>  	struct dentry *ret;
> 
>  repeat:
> -	spin_lock(&dentry->d_lock);
> +	rcu_read_lock();
>  	ret = dentry->d_parent;

Doesn't this need to be as follows?

	ret = rcu_dereference(dentry)->d_parent;

Otherwise, couldn't we end up seeing pre-initialization value for
->d_parent for a newly inserted dentry?

> -	if (!ret)
> -		goto out;
> -	if (dentry == ret) {
> -		ret->d_count++;
> -		goto out;
> -	}
> -	if (!spin_trylock(&ret->d_lock)) {
> -		spin_unlock(&dentry->d_lock);
> +	spin_lock(&ret->d_lock);

Once we do this, however, we are golden, at least for all dentry
fields protected by ->lock.  This does assume that the compiler does not
speculate the fetch that initialized the argument dentry into the critical
section, which I would sure hope would be a reasonable assumption.

> +	if (unlikely(ret != dentry->d_parent)) {
> +		spin_unlock(&ret->d_lock);
> +		rcu_read_unlock();
>  		goto repeat;
>  	}
> +	rcu_read_unlock();
>  	BUG_ON(!ret->d_count);
>  	ret->d_count++;
>  	spin_unlock(&ret->d_lock);
> -out:
> -	spin_unlock(&dentry->d_lock);
>  	return ret;
>  }
>  EXPORT_SYMBOL(dget_parent);
> @@ -601,14 +596,22 @@ static void prune_one_dentry(struct dent
>  		if (inode)
>  			spin_lock(&inode->i_lock);
>  again:
> -		spin_lock(&dentry->d_lock);
> -		if (dentry->d_parent && dentry != dentry->d_parent) {
> -			if (!spin_trylock(&dentry->d_parent->d_lock)) {
> -				spin_unlock(&dentry->d_lock);
> +		rcu_read_lock();
> +		parent = dentry->d_parent;
> +		if (parent) {
> +			spin_lock(&parent->d_lock);
> +			if (unlikely(parent != dentry->d_parent)) {
> +				spin_unlock(&parent->d_lock);
> +				rcu_read_unlock();
>  				goto again;
>  			}
> - 			parent = dentry->d_parent;
> -		}
> +			if (parent != dentry)
> +				spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
> +			else
> +				parent = NULL;
> +		} else
> +			spin_lock(&dentry->d_lock);
> +		rcu_read_unlock();
>  		dentry->d_count--;
>  		if (dentry->d_count) {
>  			if (parent)
> 
> 
> --
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