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Subject[patch 28/28] fs: consolidate dentry kill sequence
The tricky locking for disposing of a dentry is duplicated 3 times in the
dcache (dput, pruning a dentry from the LRU, and pruning its ancestors).
Consolidate them all into a single function dentry_kill.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

---
fs/dcache.c | 137 +++++++++++++++++++++++++++---------------------------------
1 file changed, 62 insertions(+), 75 deletions(-)
Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c 2010-11-17 00:52:38.000000000 +1100
+++ linux-2.6/fs/dcache.c 2010-11-17 00:52:38.000000000 +1100
@@ -244,6 +244,40 @@ static struct dentry *d_kill(struct dent
return parent;
}

+/*
+ * Finish off a dentry we've decided to kill.
+ * dentry->d_lock must be held, returns with it unlocked.
+ * If ref is non-zero, then decrement the refcount too.
+ * Returns dentry requiring refcount drop, or NULL if we're done.
+ */
+static inline struct dentry *dentry_kill(struct dentry *dentry, int ref)
+ __releases(dentry->d_lock)
+{
+ struct dentry *parent;
+
+ if (!spin_trylock(&dcache_inode_lock)) {
+relock:
+ spin_unlock(&dentry->d_lock);
+ cpu_relax();
+ return dentry; /* try again with same dentry */
+ }
+ if (IS_ROOT(dentry))
+ parent = NULL;
+ else
+ parent = dentry->d_parent;
+ if (parent && !spin_trylock(&parent->d_lock)) {
+ spin_unlock(&dcache_inode_lock);
+ goto relock;
+ }
+ if (ref)
+ dentry->d_count--;
+ /* if dentry was on the d_lru list delete it from there */
+ dentry_lru_del(dentry);
+ /* if it was on the hash then remove it */
+ __d_drop(dentry);
+ return d_kill(dentry, parent);
+}
+
/*
* This is dput
*
@@ -269,13 +303,9 @@ static struct dentry *d_kill(struct dent
* call the dentry unlink method as well as removing it from the queues and
* releasing its resources. If the parent dentries were scheduled for release
* they too may now get deleted.
- *
- * no dcache lock, please.
*/
-
void dput(struct dentry *dentry)
{
- struct dentry *parent;
if (!dentry)
return;

@@ -308,26 +338,7 @@ void dput(struct dentry *dentry)
return;

kill_it:
- if (!spin_trylock(&dcache_inode_lock)) {
-relock:
- spin_unlock(&dentry->d_lock);
- cpu_relax();
- goto repeat;
- }
- if (IS_ROOT(dentry))
- parent = NULL;
- else
- parent = dentry->d_parent;
- if (parent && !spin_trylock(&parent->d_lock)) {
- spin_unlock(&dcache_inode_lock);
- goto relock;
- }
- dentry->d_count--;
- /* if dentry was on the d_lru list delete it from there */
- dentry_lru_del(dentry);
- /* if it was on the hash (d_delete case), then remove it */
- __d_drop(dentry);
- dentry = d_kill(dentry, parent);
+ dentry = dentry_kill(dentry, 1);
if (dentry)
goto repeat;
}
@@ -528,51 +539,43 @@ void d_prune_aliases(struct inode *inode
EXPORT_SYMBOL(d_prune_aliases);

/*
- * Throw away a dentry - free the inode, dput the parent. This requires that
- * the LRU list has already been removed.
+ * Try to throw away a dentry - free the inode, dput the parent.
+ * Requires dentry->d_lock is held, and dentry->d_count == 0.
+ * Releases dentry->d_lock.
*
- * Try to prune ancestors as well. This is necessary to prevent
- * quadratic behavior of shrink_dcache_parent(), but is also expected
- * to be beneficial in reducing dentry cache fragmentation.
+ * This may fail if locks cannot be acquired no problem, just try again.
*/
-static void prune_one_dentry(struct dentry *dentry, struct dentry *parent)
+static void try_prune_one_dentry(struct dentry *dentry)
__releases(dentry->d_lock)
- __releases(parent->d_lock)
- __releases(dcache_inode_lock)
{
- __d_drop(dentry);
- dentry = d_kill(dentry, parent);
+ struct dentry *parent;

+ parent = dentry_kill(dentry, 0);
/*
- * Prune ancestors.
+ * If dentry_kill returns NULL, we have nothing more to do.
+ * if it returns the same dentry, trylocks failed. In either
+ * case, just loop again.
+ *
+ * Otherwise, we need to prune ancestors too. This is necessary
+ * to prevent quadratic behavior of shrink_dcache_parent(), but
+ * is also expected to be beneficial in reducing dentry cache
+ * fragmentation.
*/
+ if (!parent)
+ return;
+ if (parent == dentry)
+ return;
+
+ /* Prune ancestors. */
+ dentry = parent;
while (dentry) {
-relock:
spin_lock(&dentry->d_lock);
if (dentry->d_count > 1) {
dentry->d_count--;
spin_unlock(&dentry->d_lock);
- return;
- }
- if (!spin_trylock(&dcache_inode_lock)) {
-relock2:
- spin_unlock(&dentry->d_lock);
- cpu_relax();
- goto relock;
+ return;
}
-
- if (IS_ROOT(dentry))
- parent = NULL;
- else
- parent = dentry->d_parent;
- if (parent && !spin_trylock(&parent->d_lock)) {
- spin_unlock(&dcache_inode_lock);
- goto relock2;
- }
- dentry->d_count--;
- dentry_lru_del(dentry);
- __d_drop(dentry);
- dentry = d_kill(dentry, parent);
+ dentry = dentry_kill(dentry, 1);
}
}

@@ -582,8 +585,6 @@ static void shrink_dentry_list(struct li

rcu_read_lock();
while (!list_empty(list)) {
- struct dentry *parent;
-
dentry = list_entry(list->prev, struct dentry, d_lru);

/* Don't need RCU dereference because we recheck under lock */
@@ -604,24 +605,10 @@ static void shrink_dentry_list(struct li
continue;
}

- if (!spin_trylock(&dcache_inode_lock)) {
-relock:
- spin_unlock(&dentry->d_lock);
- cpu_relax();
- continue;
- }
- if (IS_ROOT(dentry))
- parent = NULL;
- else
- parent = dentry->d_parent;
- if (parent && !spin_trylock(&parent->d_lock)) {
- spin_unlock(&dcache_inode_lock);
- goto relock;
- }
- dentry_lru_del(dentry);
-
rcu_read_unlock();
- prune_one_dentry(dentry, parent);
+
+ try_prune_one_dentry(dentry);
+
rcu_read_lock();
}
rcu_read_unlock();



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