[PATCH 5.18 0890/1095] proc: fix a dentry lock race between release_task and lookup

Greg Kroah-Hartman
2022-08-15T22:57:18Z

From: Zhihao Cheng <chengzhihao1@huawei.com>

[ Upstream commit d919a1e79bac890421537cf02ae773007bf55e6b ]

Commit 7bc3e6e55acf06 ("proc: Use a list of inodes to flush from proc")
moved proc_flush_task() behind __exit_signal(). Then, process systemd can
take long period high cpu usage during releasing task in following
concurrent processes:

systemd ps
kernel_waitid stat(/proc/tgid)
do_wait filename_lookup
wait_consider_task lookup_fast
release_task
__exit_signal
__unhash_process
detach_pid
__change_pid // remove task->pid_links
d_revalidate -> pid_revalidate // 0
d_invalidate(/proc/tgid)
shrink_dcache_parent(/proc/tgid)
d_walk(/proc/tgid)
spin_lock_nested(/proc/tgid/fd)
// iterating opened fd
proc_flush_pid |
d_invalidate (/proc/tgid/fd) |
shrink_dcache_parent(/proc/tgid/fd) |
shrink_dentry_list(subdirs) ↓
shrink_lock_dentry(/proc/tgid/fd) --> race on dentry lock

Function d_invalidate() will remove dentry from hash firstly, but why does
proc_flush_pid() process dentry '/proc/tgid/fd' before dentry
'/proc/tgid'? That's because proc_pid_make_inode() adds proc inode in
reverse order by invoking hlist_add_head_rcu(). But proc should not add
any inodes under '/proc/tgid' except '/proc/tgid/task/pid', fix it by
adding inode into 'pid->inodes' only if the inode is /proc/tgid or
/proc/tgid/task/pid.

Performance regression:
Create 200 tasks, each task open one file for 50,000 times. Kill all
tasks when opened files exceed 10,000,000 (cat /proc/sys/fs/file-nr).

Before fix:
$ time killall -wq aa
real 4m40.946s # During this period, we can see 'ps' and 'systemd'
taking high cpu usage.

After fix:
$ time killall -wq aa
real 1m20.732s # During this period, we can see 'systemd' taking
high cpu usage.

Link: https://lkml.kernel.org/r/20220713130029.4133533-1-chengzhihao1@huawei.com
Fixes: 7bc3e6e55acf06 ("proc: Use a list of inodes to flush from proc")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216054
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Suggested-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/proc/base.c | 46 ++++++++++++++++++++++++++++++++++++++--------
1 file changed, 38 insertions(+), 8 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index c1031843cc6a..2f0595e8ec4a 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1885,7 +1885,7 @@ void proc_pid_evict_inode(struct proc_inode *ei)
put_pid(pid);
}

-struct inode *proc_pid_make_inode(struct super_block * sb,
+struct inode *proc_pid_make_inode(struct super_block *sb,
struct task_struct *task, umode_t mode)
{
struct inode * inode;
@@ -1914,11 +1914,6 @@ struct inode *proc_pid_make_inode(struct super_block * sb,

/* Let the pid remember us for quick removal */
ei->pid = pid;
- if (S_ISDIR(mode)) {
- spin_lock(&pid->lock);
- hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes);
- spin_unlock(&pid->lock);
- }

task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);
security_task_to_inode(task, inode);
@@ -1931,6 +1926,39 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
return NULL;
}

+/*
+ * Generating an inode and adding it into @pid->inodes, so that task will
+ * invalidate inode's dentry before being released.
+ *
+ * This helper is used for creating dir-type entries under '/proc' and
+ * '/proc/<tgid>/task'. Other entries(eg. fd, stat) under '/proc/<tgid>'
+ * can be released by invalidating '/proc/<tgid>' dentry.
+ * In theory, dentries under '/proc/<tgid>/task' can also be released by
+ * invalidating '/proc/<tgid>' dentry, we reserve it to handle single
+ * thread exiting situation: Any one of threads should invalidate its
+ * '/proc/<tgid>/task/<pid>' dentry before released.
+ */
+static struct inode *proc_pid_make_base_inode(struct super_block *sb,
+ struct task_struct *task, umode_t mode)
+{
+ struct inode *inode;
+ struct proc_inode *ei;
+ struct pid *pid;
+
+ inode = proc_pid_make_inode(sb, task, mode);
+ if (!inode)
+ return NULL;
+
+ /* Let proc_flush_pid find this directory inode */
+ ei = PROC_I(inode);
+ pid = ei->pid;
+ spin_lock(&pid->lock);
+ hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes);
+ spin_unlock(&pid->lock);
+
+ return inode;
+}
+
int pid_getattr(struct user_namespace *mnt_userns, const struct path *path,
struct kstat *stat, u32 request_mask, unsigned int query_flags)
{
@@ -3350,7 +3378,8 @@ static struct dentry *proc_pid_instantiate(struct dentry * dentry,
{
struct inode *inode;

- inode = proc_pid_make_inode(dentry->d_sb, task, S_IFDIR | S_IRUGO | S_IXUGO);
+ inode = proc_pid_make_base_inode(dentry->d_sb, task,
+ S_IFDIR | S_IRUGO | S_IXUGO);
if (!inode)
return ERR_PTR(-ENOENT);

@@ -3649,7 +3678,8 @@ static struct dentry *proc_task_instantiate(struct dentry *dentry,
struct task_struct *task, const void *ptr)
{
struct inode *inode;
- inode = proc_pid_make_inode(dentry->d_sb, task, S_IFDIR | S_IRUGO | S_IXUGO);
+ inode = proc_pid_make_base_inode(dentry->d_sb, task,
+ S_IFDIR | S_IRUGO | S_IXUGO);
if (!inode)
return ERR_PTR(-ENOENT);

--
2.35.1