From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
The cause of livelocks there is that we are taking ->d_lock on
dentry and its parent in the wrong order, forcing us to use
trylock on the parent's one. d_walk() takes them in the right
order, and unfortunately it's not hard to create a situation
when shrink_dentry_list() can't make progress since trylock
keeps failing, and shrink_dcache_parent() or check_submounts_and_drop()
keeps calling d_walk() disrupting the very shrink_dentry_list() it's
waiting for.
Solution is straightforward - if that trylock fails, let's unlock
the dentry itself and take locks in the right order. We need to
stabilize ->d_parent without holding ->d_lock, but that's doable
using RCU. And we'd better do that in the very beginning of the
loop in shrink_dentry_list(), since the checks on refcount, etc.
would need to be redone anyway.
That deals with a half of the problem - killing dentries on the
shrink list itself. Another one (dropping their parents) is
in the next commit.
locking parent is interesting - it would be easy to do rcu_read_lock(),
lock whatever we think is a parent, lock dentry itself and check
if the parent is still the right one. Except that we need to check
that *before* locking the dentry, or we are risking taking ->d_lock
out of order. Fortunately, once the D1 is locked, we can check if
D2->d_parent is equal to D1 without the need to lock D2; D2->d_parent
can start or stop pointing to D1 only under D1->d_lock, so taking
D1->d_lock is enough. In other words, the right solution is
rcu_read_lock/lock what looks like parent right now/check if it's
still our parent/rcu_read_unlock/lock the child.
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
(cherry picked from commit 046b961b45f93a92e4c70525a12f3d378bced130)
---
fs/dcache.c | 53 +++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 41 insertions(+), 12 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index 5cdd171..1c1b14c 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -529,6 +529,38 @@ dentry_kill(struct dentry *dentry, int unlock_on_failure)
return dentry; /* try again with same dentry */
}
+static inline struct dentry *lock_parent(struct dentry *dentry)
+{
+ struct dentry *parent = dentry->d_parent;
+ if (IS_ROOT(dentry))
+ return NULL;
+ if (likely(spin_trylock(&parent->d_lock)))
+ return parent;
+ spin_unlock(&dentry->d_lock);
+ rcu_read_lock();
+again:
+ parent = ACCESS_ONCE(dentry->d_parent);
+ spin_lock(&parent->d_lock);
+ /*
+ * We can't blindly lock dentry until we are sure
+ * that we won't violate the locking order.
+ * Any changes of dentry->d_parent must have
+ * been done with parent->d_lock held, so
+ * spin_lock() above is enough of a barrier
+ * for checking if it's still our child.
+ */
+ if (unlikely(parent != dentry->d_parent)) {
+ spin_unlock(&parent->d_lock);
+ goto again;
+ }
+ rcu_read_unlock();
+ if (parent != dentry)
+ spin_lock(&dentry->d_lock);
+ else
+ parent = NULL;
+ return parent;
+}
+
/*
* This is dput
*
@@ -803,6 +835,8 @@ static void shrink_dentry_list(struct list_head *list)
struct inode *inode;
dentry = list_entry(list->prev, struct dentry, d_lru);
spin_lock(&dentry->d_lock);
+ parent = lock_parent(dentry);
+
/*
* The dispose list is isolated and dentries are not accounted
* to the LRU here, so we can simply remove it from the list
@@ -816,6 +850,8 @@ static void shrink_dentry_list(struct list_head *list)
*/
if ((int)dentry->d_lockref.count > 0) {
spin_unlock(&dentry->d_lock);
+ if (parent)
+ spin_unlock(&parent->d_lock);
continue;
}
@@ -823,6 +859,8 @@ static void shrink_dentry_list(struct list_head *list)
if (unlikely(dentry->d_flags & DCACHE_DENTRY_KILLED)) {
bool can_free = dentry->d_flags & DCACHE_MAY_FREE;
spin_unlock(&dentry->d_lock);
+ if (parent)
+ spin_unlock(&parent->d_lock);
if (can_free)
dentry_free(dentry);
continue;
@@ -832,22 +870,13 @@ static void shrink_dentry_list(struct list_head *list)
if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
d_shrink_add(dentry, list);
spin_unlock(&dentry->d_lock);
+ if (parent)
+ spin_unlock(&parent->d_lock);
continue;
}
- parent = NULL;
- if (!IS_ROOT(dentry)) {
- parent = dentry->d_parent;
- if (unlikely(!spin_trylock(&parent->d_lock))) {
- if (inode)
- spin_unlock(&inode->i_lock);
- d_shrink_add(dentry, list);
- spin_unlock(&dentry->d_lock);
- continue;
- }
- }
-
__dentry_kill(dentry);
+
/*
* We need to prune ancestors too. This is necessary to prevent
* quadratic behavior of shrink_dcache_parent(), but is also
--
1.8.3.1
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