This is an updated version of the patch which adresses some issues that came
up during discussion.
The wait_on_prunes() code has moved to generic_shutdown_super() to avoid
hiding the mess in shrink_dcache_parent(). Recently updated the comments
to be more verbose about the goals of the modification.
Original patch description:
Kirill Korotaev <[email protected]> discovered a race between shrink_dcache_parent()
and shrink_dcache_memory() which leads to "Busy inodes after unmount".
When unmounting a file system shrink_dcache_parent() is racing against a
possible shrink_dcache_memory(). This might lead to the situation that
shrink_dcache_parent() is returning too early. In this situation the
super_block is destroyed before shrink_dcache_memory() could put the inode.
This patch fixes the problem through introducing a prunes counter which is
incremented when a dentry is pruned but the corresponding inoded isn't put
yet.When the prunes counter is not null, shrink_dcache_parent() is waiting and
restarting its work.
Signed-off-by: Jan Blunck <[email protected]>
Signed-off-by: Kirill Korotaev <[email protected]>
---
fs/dcache.c | 9 +++++++
fs/super.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
include/linux/fs.h | 3 ++
3 files changed, 72 insertions(+), 1 deletion(-)
Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c
+++ linux-2.6/fs/dcache.c
@@ -361,20 +361,29 @@ restart:
* This requires that the LRU list has already been
* removed.
* Called with dcache_lock, drops it and then regains.
+ *
+ * Wakes up any pending waiters (see wait_on_prunes()) if the
+ * dentry's filesystem is being umounted.
*/
static inline void prune_one_dentry(struct dentry * dentry)
{
+ struct super_block *sb = dentry->d_sb;
struct dentry * parent;
__d_drop(dentry);
list_del(&dentry->d_u.d_child);
dentry_stat.nr_dentry--; /* For d_free, below */
+ sb->s_prunes++;
dentry_iput(dentry);
parent = dentry->d_parent;
d_free(dentry);
if (parent != dentry)
dput(parent);
spin_lock(&dcache_lock);
+ sb->s_prunes--;
+ /* FIXME: this might be avoided through checking for sb->s_root */
+ if (likely(!sb->s_prunes))
+ wake_up(&sb->s_wait_prunes);
}
/**
Index: linux-2.6/fs/super.c
===================================================================
--- linux-2.6.orig/fs/super.c
+++ linux-2.6/fs/super.c
@@ -80,6 +80,8 @@ static struct super_block *alloc_super(v
sema_init(&s->s_dquot.dqio_sem, 1);
sema_init(&s->s_dquot.dqonoff_sem, 1);
init_rwsem(&s->s_dquot.dqptr_sem);
+ s->s_prunes = 0;
+ init_waitqueue_head(&s->s_wait_prunes);
init_waitqueue_head(&s->s_wait_unfrozen);
s->s_maxbytes = MAX_NON_LFS;
s->dq_op = sb_dquot_ops;
@@ -213,6 +215,51 @@ static int grab_super(struct super_block
return 0;
}
+/*
+ * A special version of wait_event(!sb->s_prunes) which takes the dcache_lock
+ * when checking the condition and gives feedback if we have waited for
+ * remaining prunes.
+ *
+ * While we are waiting for pruned dentries to iput() their inode, the
+ * sb->s_prunes count is none zero. Since the s_prunes counter is modified
+ * by prune_one_dentry() under dcache_lock, either the reference count on the
+ * parent dentry is wrong (and therefore it isn't on the lru-list yet) and we
+ * are waiting because s_prunes != 0 or the reference count is correct (and the
+ * parent dentry might be found by select_parent()) and the s_prunes == 0. This
+ * is the reason for that we have to restart loop on select_parent() and
+ * prune_dcache() as long as we are waiting on pruned dentries here (see the
+ * usage of wait_on_prunes() in generic_shutdown_super()). This is not
+ * live-locking because we only call wait_on_prunes() if we are umounting the
+ * filesystem (sb->s_root == NULL), therefore noone is having a reference to
+ * the vfsmount structure anymore and hence shouldn't have a reference to a
+ * dentry.
+ */
+static int wait_on_prunes(struct super_block *sb)
+{
+ DEFINE_WAIT(wait);
+ int prunes_remaining = 0;
+
+#ifdef DCACHE_DEBUG
+ printk(KERN_DEBUG "%s: waiting for %d prunes\n", __FUNCTION__,
+ sb->s_prunes);
+#endif
+
+ spin_lock(&dcache_lock);
+ for (;;) {
+ prepare_to_wait(&sb->s_wait_prunes, &wait,
+ TASK_UNINTERRUPTIBLE);
+ if (!sb->s_prunes)
+ break;
+ spin_unlock(&dcache_lock);
+ schedule();
+ prunes_remaining = 1;
+ spin_lock(&dcache_lock);
+ }
+ spin_unlock(&dcache_lock);
+ finish_wait(&sb->s_wait_prunes, &wait);
+ return prunes_remaining;
+}
+
/**
* generic_shutdown_super - common helper for ->kill_sb()
* @sb: superblock to kill
@@ -230,8 +277,20 @@ void generic_shutdown_super(struct super
if (root) {
sb->s_root = NULL;
- shrink_dcache_parent(root);
shrink_dcache_anon(&sb->s_anon);
+
+ /*
+ * If we have waited for a pruned dentry which parent wasn't
+ * unhashed yet, that parent will end up on the lru-list.
+ * Therefore, we need to run shrink_dcache_parent() again
+ * after we waited for pruned dentries in wait_on_prunes().
+ *
+ * NOTE: Also read the comment on wait_on_prunes() above.
+ */
+ do {
+ shrink_dcache_parent(root);
+ } while(wait_on_prunes(sb));
+
dput(root);
fsync_super(sb);
lock_super(sb);
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h
+++ linux-2.6/include/linux/fs.h
@@ -836,6 +836,9 @@ struct super_block {
struct list_head s_instances;
struct quota_info s_dquot; /* Diskquota specific options */
+ unsigned int s_prunes; /* protected by dcache_lock */
+ wait_queue_head_t s_wait_prunes;
+
int s_frozen;
wait_queue_head_t s_wait_unfrozen;
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