Re: [PATCH 2/3] fs/locks: Remove fl_nspid

[Jeff Layton]

On Tue, 2017-06-06 at 14:00 -0400, Jeff Layton wrote:
> On Tue, 2017-06-06 at 13:19 -0400, Benjamin Coddington wrote:
> > Since commit c69899a17ca4 "NFSv4: Update of VFS byte range lock must be
> > atomic with the stateid update", NFSv4 has been inserting locks in rpciod
> > worker context.  The result is that the file_lock's fl_nspid is the
> > kworker's pid instead of the original userspace pid.
> > 
> > The fl_nspid is only used to represent the namespaced virtual pid number
> > when displaying locks or returning from F_GETLK.  There's no reason to set
> > it for every inserted lock, since we can usually just look it up from
> > fl_pid.  So, instead of looking up and holding struct pid for every lock,
> > let's just look up the virtual pid number from fl_pid when it is needed.
> > That means we can remove fl_nspid entirely.
> > 
> 
> With this set, I think we ought to codify that the stored pid must be
> relative 

...to the init_pid_ns. Let's make that clear in the comments for
filesystem authors.

> 
> 
> > Signed-off-by: Benjamin Coddington <bcodding@xxxxxxxxxx>
> > ---
> >  fs/locks.c         | 58 ++++++++++++++++++++++++++++++++----------------------
> >  include/linux/fs.h |  1 -
> >  2 files changed, 35 insertions(+), 24 deletions(-)
> > 
> > diff --git a/fs/locks.c b/fs/locks.c
> > index d7daa6c8932f..104398ccc9b9 100644
> > --- a/fs/locks.c
> > +++ b/fs/locks.c
> > @@ -733,7 +733,6 @@ static void locks_wake_up_blocks(struct file_lock *blocker)
> >  static void
> >  locks_insert_lock_ctx(struct file_lock *fl, struct list_head *before)
> >  {
> > -	fl->fl_nspid = get_pid(task_tgid(current));
> >  	list_add_tail(&fl->fl_list, before);
> >  	locks_insert_global_locks(fl);
> >  }
> > @@ -743,10 +742,6 @@ locks_unlink_lock_ctx(struct file_lock *fl)
> >  {
> >  	locks_delete_global_locks(fl);
> >  	list_del_init(&fl->fl_list);
> > -	if (fl->fl_nspid) {
> > -		put_pid(fl->fl_nspid);
> > -		fl->fl_nspid = NULL;
> > -	}
> >  	locks_wake_up_blocks(fl);
> >  }
> >  
> > @@ -823,8 +818,6 @@ posix_test_lock(struct file *filp, struct file_lock *fl)
> >  	list_for_each_entry(cfl, &ctx->flc_posix, fl_list) {
> >  		if (posix_locks_conflict(fl, cfl)) {
> >  			locks_copy_conflock(fl, cfl);
> > -			if (cfl->fl_nspid)
> > -				fl->fl_pid = pid_vnr(cfl->fl_nspid);
> >  			goto out;
> >  		}
> >  	}
> > @@ -2048,6 +2041,31 @@ int vfs_test_lock(struct file *filp, struct file_lock *fl)
> >  }
> >  EXPORT_SYMBOL_GPL(vfs_test_lock);
> >  
> > +/**
> > + * locks_translate_pid - translate a pid number into a namespace
> > + * @nr: The pid number in the init_pid_ns
> > + * @ns: The namespace into which the pid should be translated
> > + *
> > + * Used to tranlate a fl_pid into a namespace virtual pid number
> > + */
> > +static pid_t locks_translate_pid(int init_nr, struct pid_namespace *ns)
> > +{
> > +	pid_t vnr = 0;
> > +	struct task_struct *task;
> > +
> > +	rcu_read_lock();
> > +	task = find_task_by_pid_ns(init_nr, &init_pid_ns);
> > +	if (task)
> > +		get_task_struct(task);
> > +	rcu_read_unlock();
> 
> Is that safe? What prevents get_task_struct from doing a 0->1 transition
> there after the task usage count has already gone 1->0 and is on its way
> to being freed?
> 
> > +	if (!task)
> > +		goto out;
> > +	vnr = task_pid_nr_ns(task, ns);
> > +	put_task_struct(task);
> > +out:
> > +	return vnr;
> > +}


Now that I look, I think it might be best to just do all of that under
the rcu_read_lock and don't muck with the refcount at all. So something
like:

+static pid_t locks_translate_pid(int init_nr, struct pid_namespace *ns)
+{
+ÂÂÂÂÂÂÂpid_t vnr = 0;
+ÂÂÂÂÂÂÂstruct task_struct *task;
+
+ÂÂÂÂÂÂÂrcu_read_lock();
+ÂÂÂÂÂÂÂtask = find_task_by_pid_ns(init_nr, &init_pid_ns);
+ÂÂÂÂÂÂÂif (task)
+ÂÂÂÂÂÂÂ        vnr = task_pid_nr_ns(task, ns);
+ÂÂÂÂÂÂÂrcu_read_unlock();
+ÂÂÂÂÂÂÂreturn vnr;
+}

That should be fine since task_pid_nr_ns pretty much does all of its
work under the rcu_read_lock anyway.


> >  static int posix_lock_to_flock(struct flock *flock, struct file_lock *fl)
> >  {
> >  	flock->l_pid = IS_OFDLCK(fl) ? -1 : fl->fl_pid;
> > @@ -2584,22 +2602,16 @@ static void lock_get_status(struct seq_file *f, struct file_lock *fl,
> >  {
> >  	struct inode *inode = NULL;
> >  	unsigned int fl_pid;
> > +	struct pid_namespace *proc_pidns = file_inode(f->file)->i_sb->s_fs_info;
> >  
> > -	if (fl->fl_nspid) {
> > -		struct pid_namespace *proc_pidns = file_inode(f->file)->i_sb->s_fs_info;
> > -
> > -		/* Don't let fl_pid change based on who is reading the file */
> > -		fl_pid = pid_nr_ns(fl->fl_nspid, proc_pidns);
> > -
> > -		/*
> > -		 * If there isn't a fl_pid don't display who is waiting on
> > -		 * the lock if we are called from locks_show, or if we are
> > -		 * called from __show_fd_info - skip lock entirely
> > -		 */
> > -		if (fl_pid == 0)
> > -			return;
> > -	} else
> > -		fl_pid = fl->fl_pid;
> > +	fl_pid = locks_translate_pid(fl->fl_pid, proc_pidns);
> > +	/*
> > +	 * If there isn't a fl_pid don't display who is waiting on
> > +	 * the lock if we are called from locks_show, or if we are
> > +	 * called from __show_fd_info - skip lock entirely
> > +	 */
> > +	if (fl_pid == 0)
> > +		return;
> >  
> >  	if (fl->fl_file != NULL)
> >  		inode = locks_inode(fl->fl_file);
> > @@ -2674,7 +2686,7 @@ static int locks_show(struct seq_file *f, void *v)
> >  
> >  	fl = hlist_entry(v, struct file_lock, fl_link);
> >  
> > -	if (fl->fl_nspid && !pid_nr_ns(fl->fl_nspid, proc_pidns))
> > +	if (locks_translate_pid(fl->fl_pid, proc_pidns) == 0)
> >  		return 0;
> >  
> >  	lock_get_status(f, fl, iter->li_pos, "");
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index aa4affb38c39..b013fac515f7 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -984,7 +984,6 @@ struct file_lock {
> >  	unsigned char fl_type;
> >  	unsigned int fl_pid;
> >  	int fl_link_cpu;		/* what cpu's list is this on? */
> > -	struct pid *fl_nspid;
> >  	wait_queue_head_t fl_wait;
> >  	struct file *fl_file;
> >  	loff_t fl_start;
> 
> 

-- 
Jeff Layton <jlayton@xxxxxxxxxx>