Messages in this thread | | | Date | Thu, 8 Jul 2010 00:35:56 +1000 | From | Nick Piggin <> | Subject | Re: [patch 24/52] fs: dcache reduce d_parent locking |
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Hi Paul,
Sorry I had left this in my postponed folder while rechecking your questions and forgot about it :P
On Mon, Jun 28, 2010 at 02:50:13PM -0700, Paul E. McKenney wrote: > 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?
I don't think so. The child's dentry memory should be guaranteed to be post-initialized at the point it is passed to dget_parent, becase we've to have a stable refcount on it at that point. Ie. if it was pulled from an RCU list, it should already have been rcu dereferenced by now.
So ->d_parent should be a valid pointer with lifetime guarantee provided by RCU -- so enough to take the spinlock and recheck.
> > > - 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.
Yes I think the above fact that we have a "good" ref on the dentry should prevent this.
Thanks, Nick
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