diff options
author | Al Viro <viro@zeniv.linux.org.uk> | 2014-05-29 08:54:52 -0400 |
---|---|---|
committer | Al Viro <viro@zeniv.linux.org.uk> | 2014-05-30 11:03:21 -0400 |
commit | 046b961b45f93a92e4c70525a12f3d378bced130 (patch) | |
tree | 25c7bf216122f3c49c5ed0516b6b4b260316d85d | |
parent | ff2fde9929feb2aef45377ce56b8b12df85dda69 (diff) |
shrink_dentry_list(): take parent's ->d_lock earlier
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@zeniv.linux.org.uk>
-rw-r--r-- | fs/dcache.c | 53 |
1 files changed, 41 insertions, 12 deletions
diff --git a/fs/dcache.c b/fs/dcache.c index c23f78a9d156..d54a99baf4f3 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -530,6 +530,38 @@ failed: 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 * @@ -804,6 +836,8 @@ 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 @@ -817,6 +851,8 @@ 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; } @@ -824,6 +860,8 @@ 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; @@ -833,22 +871,13 @@ 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 |