Re: [PATCH v5] proc: Fix a dentry lock race between release_task and lookup

[Brian Foster]

On Wed, Jul 13, 2022 at 09:00:29PM +0800, Zhihao Cheng wrote:
> 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.
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> Suggested-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---

LGTM, and thanks for the tweaks:

Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>

>  v1->v2: Add new helper proc_pid_make_base_inode that performs the extra
> 	 work of adding to the pid->list.
>  v2->v3: Add performance regression in commit message.
>  v3->v4: Make proc_pid_make_base_inode() static
>  v4->v5: Add notes to explain what proc_pid_make_base_inode() does
>  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 8dfa36a99c74..93f7e3d971e4 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)
>  {
> @@ -3369,7 +3397,8 @@ 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);
>  
> @@ -3671,7 +3700,8 @@ 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);
>  
> -- 
> 2.31.1
>