Re: [PATCH RFC] proc: Fix a dentry lock race between release_task and lookup

[Eric W. Biederman]

Zhihao Cheng <chengzhihao1@xxxxxxxxxx> writes:

> 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/pid)
>   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/pid)
>                                        shrink_dcache_parent(/proc/pid)
>                                          d_walk(/proc/pid)
>                                            spin_lock_nested(/proc/pid/fd)
>                                            // iterating opened fd
>         proc_flush_pid                                    |
>            d_invalidate (/proc/pid/fd)                    |
>               shrink_dcache_parent(/proc/pid/fd)          |
>                 shrink_dentry_list(subdirs)               ↓
>                   shrink_lock_dentry(/proc/pid/fd) ---> race on dentry lock
>
> Function d_invalidate() will remove dentry from hash firstly, but why does
> proc_flush_pid() process dentry '/proc/pid/fd' before dentry '/proc/pid'?
> 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/pid' except '/proc/pid/task/pid', fix it by adding inode into
> 'pid->inodes' only if the inode is /proc/pid or /proc/pid/task/pid.

If I understand correctly you are saying that under some circumstances
this code runs slow, and you are proposing an optimization.

That optimization is to change the content of the pid->inodes list
from all directories under that pid, to just the /proc/<tgid> and
/proc/<tgid>/task/<pid>.

The justification being that d_invalidate on the parent directory will
invalidate all children.  So only those two directories are interesting
from a d_invalidate point of view.

That seems like a valid optimization.

This could also count as a regression fix if you can show how the
performance changed poorly when the pid->inodes change was introduced
and how the performance improves with your change.   I currently only
see that you hit a pathological case and you are correcting it.

As for the actual code change I think it would be better to
remove the code from proc_pid_make_inode and make a helper
proc_pid_make_base_inode that performs the extra work of
adding to the pid->list.  Not adding a flag makes the code
easier to follow.

Something like the code below.  

diff --git a/fs/proc/base.c b/fs/proc/base.c
index d654ce7150fd..9d025e70ddc3 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1915,11 +1915,6 @@ 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);
@@ -1932,6 +1927,27 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
 	return NULL;
 }
 
+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)
 {
@@ -3351,7 +3367,7 @@ 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);
 
@@ -3650,7 +3666,7 @@ 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);
 

Eric