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From: Paul E. McKenney on 28 Jun 2010 18:00 On Thu, Jun 24, 2010 at 01:02:36PM +1000, npiggin(a)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(a)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) > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo(a)vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Nick Piggin on 7 Jul 2010 10:40 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(a)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(a)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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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