From df3d0bbcdb2cafa23a70223d806655bd37e64a9b Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Mon, 2 Sep 2013 11:29:22 -0700 Subject: vfs: use lockref_get_not_zero() for optimistic lockless dget_parent() A valid parent pointer is always going to have a non-zero reference count, but if we look up the parent optimistically without locking, we have to protect against the (very unlikely) race against renaming changing the parent from under us. We do that by using lockref_get_not_zero(), and then re-checking the parent pointer after getting a valid reference. [ This is a re-implementation of a chunk from the original patch by Waiman Long: "dcache: Enable lockless update of dentry's refcount". I've completely rewritten the patch-series and split it up, but I'm attributing this part to Waiman as it's close enough to his earlier patch - Linus ] Signed-off-by: Waiman Long Signed-off-by: Linus Torvalds --- fs/dcache.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) (limited to 'fs') diff --git a/fs/dcache.c b/fs/dcache.c index b949af850cd6..2d244227999d 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -611,8 +611,23 @@ static inline void __dget(struct dentry *dentry) struct dentry *dget_parent(struct dentry *dentry) { + int gotref; struct dentry *ret; + /* + * Do optimistic parent lookup without any + * locking. + */ + rcu_read_lock(); + ret = ACCESS_ONCE(dentry->d_parent); + gotref = lockref_get_not_zero(&ret->d_lockref); + rcu_read_unlock(); + if (likely(gotref)) { + if (likely(ret == ACCESS_ONCE(dentry->d_parent))) + return ret; + dput(ret); + } + repeat: /* * Don't need rcu_dereference because we re-check it was correct under -- cgit v1.2.3-58-ga151