Re: [RFC PATCH bpf-next 08/16] bpf: add task and task/file targets

Alexei Starovoitov
2020-04-10T21:31:44Z

On Thu, Apr 09, 2020 at 11:19:10PM -0700, Yonghong Song wrote:
>
>
> On 4/9/20 8:22 PM, Alexei Starovoitov wrote:
> > On Wed, Apr 08, 2020 at 04:25:29PM -0700, Yonghong Song wrote:
> > > +
> > > + spin_lock(&files->file_lock);
> > > + for (; sfd < files_fdtable(files)->max_fds; sfd++) {
> > > + struct file *f;
> > > +
> > > + f = fcheck_files(files, sfd);
> > > + if (!f)
> > > + continue;
> > > +
> > > + *fd = sfd;
> > > + get_file(f);
> > > + spin_unlock(&files->file_lock);
> > > + return f;
> > > + }
> > > +
> > > + /* the current task is done, go to the next task */
> > > + spin_unlock(&files->file_lock);
> > > + put_files_struct(files);
> >
> > I think spin_lock is unnecessary.
> > It's similarly unnecessary in bpf_task_fd_query().
> > Take a look at proc_readfd_common() in fs/proc/fd.c.
> > It only needs rcu_read_lock() to iterate fd array.
>
> I see. I was looking at function seq_show() at fs/proc/fd.c,
>
> ...
> spin_lock(&files->file_lock);
> file = fcheck_files(files, fd);
> if (file) {
> struct fdtable *fdt = files_fdtable(files);
>
> f_flags = file->f_flags;
> if (close_on_exec(fd, fdt))
> f_flags |= O_CLOEXEC;
>
> get_file(file);
> ret = 0;
> }
> spin_unlock(&files->file_lock);
> put_files_struct(files);
> ...
>
> I guess here spin_lock is needed due to close_on_exec().

Right. fdr->close_on_exec array is not rcu protected and needs that spin_lock.

> Will use rcu_read_lock() mechanism then.

Thanks!