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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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