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SubjectRe: [patch 24/52] fs: dcache reduce d_parent locking
On Thu, Jun 24, 2010 at 01:02:36PM +1000, npiggin@suse.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@suse.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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