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

[Zhihao Cheng]

在 2022/7/12 22:16, Brian Foster 写道:
> 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?

Yes. It could happend too. There will be many dentries under 
/proc/<tgid>/task when there are many tasks under same thread group.

We must put /proc/<tgid>/task into pid->inodes, because we have to 
handle single thread exiting situation: Any one of threads should 
invalidate its /proc/<tgid>/task/<pid> dentry before begin released. You 
may refer to the function proc_flush_task_mnt() before commit 
7bc3e6e55acf06 ("proc: Use a list of inodes to flush from proc").

> ...
> > 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>
> > ---
> >   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..

Thanks for advice, I will add some notes in v5.
> 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
>
> .