Re: [External] Re: [PATCH] files: Use rcu lock to get the file structures for better performance

[Muchun Song <songmuchun@bytedance.com>]

On Fri, May 22, 2020 at 12:47 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, May 21, 2020 at 08:38:35PM +0800, Muchun Song wrote:
> > +++ b/fs/proc/fd.c
> > @@ -34,19 +34,27 @@ static int seq_show(struct seq_file *m, void *v)
> >       if (files) {
> >               unsigned int fd = proc_fd(m->private);
> >
> > -             spin_lock(&files->file_lock);
> > +             rcu_read_lock();
> > +again:
> >               file = fcheck_files(files, fd);
> >               if (file) {
> > -                     struct fdtable *fdt = files_fdtable(files);
> > +                     struct fdtable *fdt;
> > +
> > +                     if (!get_file_rcu(file)) {
> > +                             /*
> > +                              * we loop to catch the new file (or NULL
> > +                              * pointer).
> > +                              */
> > +                             goto again;
> > +                     }
> >
> > +                     fdt = files_fdtable(files);
>
> This is unusual, and may not be safe.
>
> fcheck_files() loads files->fdt.  Then it loads file from fdt->fd[].
> Now you're loading files->fdt again here, and it could have been changed
> by another thread expanding the fd table.
>
> You have to write a changelog which convinces me you've thought about
> this race and that it's safe.  Because I don't think you even realise
> it's a possibility at this point.

Thanks for your review, it is a problem. I can fix it.

>
> > @@ -160,14 +168,23 @@ static int proc_fd_link(struct dentry *dentry, struct path *path)
> >               unsigned int fd = proc_fd(d_inode(dentry));
> >               struct file *fd_file;
> >
> > -             spin_lock(&files->file_lock);
> > +             rcu_read_lock();
> > +again:
> >               fd_file = fcheck_files(files, fd);
> >               if (fd_file) {
> > +                     if (!get_file_rcu(fd_file)) {
> > +                             /*
> > +                              * we loop to catch the new file
> > +                              * (or NULL pointer).
> > +                              */
> > +                             goto again;
> > +                     }
> >                       *path = fd_file->f_path;
> >                       path_get(&fd_file->f_path);
> > +                     fput(fd_file);
> >                       ret = 0;
> >               }
> > -             spin_unlock(&files->file_lock);
> > +             rcu_read_unlock();
>
> Why is it an improvement to increment/decrement the refcount on the
> struct file here, rather than take/release the spinlock?
>

lock-free vs spinlock.

Do you think spinlock would be better than the lock-free method?
Actually I prefer the rcu lock.

-- 
Yours,
Muchun