From: Hao Luo haoluo@google.com
[ Upstream commit 1a702dc88e150487c9c173a249b3d236498b9183 ]
Previously the protection of kernfs_pr_cont_buf was piggy backed by rename_lock, which means that pr_cont() needs to be protected under rename_lock. This can cause potential circular lock dependencies.
If there is an OOM, we have the following call hierarchy:
-> cpuset_print_current_mems_allowed() -> pr_cont_cgroup_name() -> pr_cont_kernfs_name()
pr_cont_kernfs_name() will grab rename_lock and call printk. So we have the following lock dependencies:
kernfs_rename_lock -> console_sem
Sometimes, printk does a wakeup before releasing console_sem, which has the dependence chain:
console_sem -> p->pi_lock -> rq->lock
Now, imagine one wants to read cgroup_name under rq->lock, for example, printing cgroup_name in a tracepoint in the scheduler code. They will be holding rq->lock and take rename_lock:
rq->lock -> kernfs_rename_lock
Now they will deadlock.
A prevention to this circular lock dependency is to separate the protection of pr_cont_buf from rename_lock. In principle, rename_lock is to protect the integrity of cgroup name when copying to buf. Once pr_cont_buf has got its content, rename_lock can be dropped. So it's safe to drop rename_lock after kernfs_name_locked (and kernfs_path_from_node_locked) and rely on a dedicated pr_cont_lock to protect pr_cont_buf.
Acked-by: Tejun Heo tj@kernel.org Signed-off-by: Hao Luo haoluo@google.com Link: https://lore.kernel.org/r/20220516190951.3144144-1-haoluo@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- fs/kernfs/dir.c | 31 +++++++++++++++++++------------ 1 file changed, 19 insertions(+), 12 deletions(-)
diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index 8697b750b1c9..fa2dee322ee9 100644 --- a/fs/kernfs/dir.c +++ b/fs/kernfs/dir.c @@ -20,7 +20,15 @@
DEFINE_MUTEX(kernfs_mutex); static DEFINE_SPINLOCK(kernfs_rename_lock); /* kn->parent and ->name */ -static char kernfs_pr_cont_buf[PATH_MAX]; /* protected by rename_lock */ +/* + * Don't use rename_lock to piggy back on pr_cont_buf. We don't want to + * call pr_cont() while holding rename_lock. Because sometimes pr_cont() + * will perform wakeups when releasing console_sem. Holding rename_lock + * will introduce deadlock if the scheduler reads the kernfs_name in the + * wakeup path. + */ +static DEFINE_SPINLOCK(kernfs_pr_cont_lock); +static char kernfs_pr_cont_buf[PATH_MAX]; /* protected by pr_cont_lock */ static DEFINE_SPINLOCK(kernfs_idr_lock); /* root->ino_idr */
#define rb_to_kn(X) rb_entry((X), struct kernfs_node, rb) @@ -229,12 +237,12 @@ void pr_cont_kernfs_name(struct kernfs_node *kn) { unsigned long flags;
- spin_lock_irqsave(&kernfs_rename_lock, flags); + spin_lock_irqsave(&kernfs_pr_cont_lock, flags);
- kernfs_name_locked(kn, kernfs_pr_cont_buf, sizeof(kernfs_pr_cont_buf)); + kernfs_name(kn, kernfs_pr_cont_buf, sizeof(kernfs_pr_cont_buf)); pr_cont("%s", kernfs_pr_cont_buf);
- spin_unlock_irqrestore(&kernfs_rename_lock, flags); + spin_unlock_irqrestore(&kernfs_pr_cont_lock, flags); }
/** @@ -248,10 +256,10 @@ void pr_cont_kernfs_path(struct kernfs_node *kn) unsigned long flags; int sz;
- spin_lock_irqsave(&kernfs_rename_lock, flags); + spin_lock_irqsave(&kernfs_pr_cont_lock, flags);
- sz = kernfs_path_from_node_locked(kn, NULL, kernfs_pr_cont_buf, - sizeof(kernfs_pr_cont_buf)); + sz = kernfs_path_from_node(kn, NULL, kernfs_pr_cont_buf, + sizeof(kernfs_pr_cont_buf)); if (sz < 0) { pr_cont("(error)"); goto out; @@ -265,7 +273,7 @@ void pr_cont_kernfs_path(struct kernfs_node *kn) pr_cont("%s", kernfs_pr_cont_buf);
out: - spin_unlock_irqrestore(&kernfs_rename_lock, flags); + spin_unlock_irqrestore(&kernfs_pr_cont_lock, flags); }
/** @@ -850,13 +858,12 @@ static struct kernfs_node *kernfs_walk_ns(struct kernfs_node *parent,
lockdep_assert_held(&kernfs_mutex);
- /* grab kernfs_rename_lock to piggy back on kernfs_pr_cont_buf */ - spin_lock_irq(&kernfs_rename_lock); + spin_lock_irq(&kernfs_pr_cont_lock);
len = strlcpy(kernfs_pr_cont_buf, path, sizeof(kernfs_pr_cont_buf));
if (len >= sizeof(kernfs_pr_cont_buf)) { - spin_unlock_irq(&kernfs_rename_lock); + spin_unlock_irq(&kernfs_pr_cont_lock); return NULL; }
@@ -868,7 +875,7 @@ static struct kernfs_node *kernfs_walk_ns(struct kernfs_node *parent, parent = kernfs_find_ns(parent, name, ns); }
- spin_unlock_irq(&kernfs_rename_lock); + spin_unlock_irq(&kernfs_pr_cont_lock);
return parent; }