Re: [PATCH v4] proc: Fix a dentry lock race between release_task and lookup

[Brian Foster <bfoster@redhat.com>]

On Wed, Jun 01, 2022 at 02:23:32PM +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
> 

Curious... can this same sort of thing happen with /proc/<tgid>/task if
that dir similarly has a lot of dentries?

...
> 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>
> ---
>  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
>  fs/proc/base.c | 34 ++++++++++++++++++++++++++--------
>  1 file changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index c1031843cc6a..d884933950fd 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
...
> @@ -1931,6 +1926,27 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
>  	return NULL;
>  }
>  
> +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;
> +}
> +

Somewhat related to the question above.. it would be nice if this
wrapper had a line or two comment above it that explained when it should
or shouldn't be used over the underlying function (for example, why or
why not include /proc/<tgid>/task?). Otherwise the patch overall seems
reasonable to me..

Brian

>  int pid_getattr(struct user_namespace *mnt_userns, const struct path *path,
>  		struct kstat *stat, u32 request_mask, unsigned int query_flags)
>  {
> @@ -3350,7 +3366,8 @@ 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 +3666,8 @@ 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
>