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Subject[CFT] unify dentry_iput() and dentry_unlink_inode()
[bringing back an old tangent from a thread back in February]

There is a lot of duplication between dentry_unlink_inode() and dentry_iput().
The only real difference is that dentry_unlink_inode() bumps ->d_seq and
dentry_iput() doesn't. The argument of the latter is known to have been
unhashed, so anybody who might've found it in RCU lookup would already be
doomed to a ->d_seq mismatch. And we want to avoid pointless smp_rmb() there.

This patch makes dentry_unlink_inode() bump ->d_seq only for hashed dentries.
It's safe (d_delete() calls that sucker only if we are holding the only
reference to dentry, so rehash is not going to happen) and it allows
to use dentry_unlink_inode() in __dentry_kill() and get rid of dentry_iput().

The interesting question here is profiling; it *is* a hot path, and extra
conditional jumps in there might or might not be painful.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---

diff --git a/fs/dcache.c b/fs/dcache.c
index f9c63c1..fe7cde2 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -335,44 +335,21 @@ static inline void dentry_rcuwalk_invalidate(struct dentry *dentry)

/*
* Release the dentry's inode, using the filesystem
- * d_iput() operation if defined. Dentry has no refcount
- * and is unhashed.
- */
-static void dentry_iput(struct dentry * dentry)
- __releases(dentry->d_lock)
- __releases(dentry->d_inode->i_lock)
-{
- struct inode *inode = dentry->d_inode;
- if (inode) {
- __d_clear_type_and_inode(dentry);
- hlist_del_init(&dentry->d_u.d_alias);
- spin_unlock(&dentry->d_lock);
- spin_unlock(&inode->i_lock);
- if (!inode->i_nlink)
- fsnotify_inoderemove(inode);
- if (dentry->d_op && dentry->d_op->d_iput)
- dentry->d_op->d_iput(dentry, inode);
- else
- iput(inode);
- } else {
- spin_unlock(&dentry->d_lock);
- }
-}
-
-/*
- * Release the dentry's inode, using the filesystem
- * d_iput() operation if defined. dentry remains in-use.
+ * d_iput() operation if defined.
*/
static void dentry_unlink_inode(struct dentry * dentry)
__releases(dentry->d_lock)
__releases(dentry->d_inode->i_lock)
{
struct inode *inode = dentry->d_inode;
+ bool hashed = !d_unhashed(dentry);

- raw_write_seqcount_begin(&dentry->d_seq);
+ if (hashed)
+ raw_write_seqcount_begin(&dentry->d_seq);
__d_clear_type_and_inode(dentry);
hlist_del_init(&dentry->d_u.d_alias);
- raw_write_seqcount_end(&dentry->d_seq);
+ if (hashed)
+ raw_write_seqcount_end(&dentry->d_seq);
spin_unlock(&dentry->d_lock);
spin_unlock(&inode->i_lock);
if (!inode->i_nlink)
@@ -540,12 +517,10 @@ static void __dentry_kill(struct dentry *dentry)
dentry->d_flags |= DCACHE_DENTRY_KILLED;
if (parent)
spin_unlock(&parent->d_lock);
- dentry_iput(dentry);
- /*
- * dentry_iput drops the locks, at which point nobody (except
- * transient RCU lookups) can reach this dentry.
- */
- BUG_ON(dentry->d_lockref.count > 0);
+ if (dentry->d_inode)
+ dentry_unlink_inode(dentry);
+ else
+ spin_unlock(&dentry->d_lock);
this_cpu_dec(nr_dentry);
if (dentry->d_op && dentry->d_op->d_release)
dentry->d_op->d_release(dentry);
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