From ba16b2846a8c6965d0d35be3968bc10f6277812d Mon Sep 17 00:00:00 2001 From: Shaohua Li Date: Wed, 12 Jul 2017 11:49:48 -0700 Subject: kernfs: add an API to get kernfs node from inode number Add an API to get kernfs node from inode number. We will need this to implement exportfs operations. This API will be used in blktrace too later, so it should be as fast as possible. To make the API lock free, kernfs node is freed in RCU context. And we depend on kernfs_node count/ino number to filter out stale kernfs nodes. Acked-by: Tejun Heo Acked-by: Greg Kroah-Hartman Signed-off-by: Shaohua Li Signed-off-by: Jens Axboe --- fs/kernfs/dir.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 57 insertions(+) (limited to 'fs/kernfs/dir.c') diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index 33f711f6b86e..7be37c838007 100644 --- a/fs/kernfs/dir.c +++ b/fs/kernfs/dir.c @@ -508,6 +508,10 @@ void kernfs_put(struct kernfs_node *kn) struct kernfs_node *parent; struct kernfs_root *root; + /* + * kernfs_node is freed with ->count 0, kernfs_find_and_get_node_by_ino + * depends on this to filter reused stale node + */ if (!kn || !atomic_dec_and_test(&kn->count)) return; root = kernfs_root(kn); @@ -649,6 +653,11 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root, kn->ino = ret; kn->generation = gen; + /* + * set ino first. This barrier is paired with atomic_inc_not_zero in + * kernfs_find_and_get_node_by_ino + */ + smp_mb__before_atomic(); atomic_set(&kn->count, 1); atomic_set(&kn->active, KN_DEACTIVATED_BIAS); RB_CLEAR_NODE(&kn->rb); @@ -680,6 +689,54 @@ struct kernfs_node *kernfs_new_node(struct kernfs_node *parent, return kn; } +/* + * kernfs_find_and_get_node_by_ino - get kernfs_node from inode number + * @root: the kernfs root + * @ino: inode number + * + * RETURNS: + * NULL on failure. Return a kernfs node with reference counter incremented + */ +struct kernfs_node *kernfs_find_and_get_node_by_ino(struct kernfs_root *root, + unsigned int ino) +{ + struct kernfs_node *kn; + + rcu_read_lock(); + kn = idr_find(&root->ino_idr, ino); + if (!kn) + goto out; + + /* + * Since kernfs_node is freed in RCU, it's possible an old node for ino + * is freed, but reused before RCU grace period. But a freed node (see + * kernfs_put) or an incompletedly initialized node (see + * __kernfs_new_node) should have 'count' 0. We can use this fact to + * filter out such node. + */ + if (!atomic_inc_not_zero(&kn->count)) { + kn = NULL; + goto out; + } + + /* + * The node could be a new node or a reused node. If it's a new node, + * we are ok. If it's reused because of RCU (because of + * SLAB_TYPESAFE_BY_RCU), the __kernfs_new_node always sets its 'ino' + * before 'count'. So if 'count' is uptodate, 'ino' should be uptodate, + * hence we can use 'ino' to filter stale node. + */ + if (kn->ino != ino) + goto out; + rcu_read_unlock(); + + return kn; +out: + rcu_read_unlock(); + kernfs_put(kn); + return NULL; +} + /** * kernfs_add_one - add kernfs_node to parent without warning * @kn: kernfs_node to be added -- cgit v1.2.3-58-ga151