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

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Date:   Fri, 10 Apr 2020 14:33:33 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [RFC PATCH bpf-next 08/16] bpf: add task and task/file targets

On Fri, Apr 10, 2020 at 2:31 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> 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.

Actually. I'll take it back. fdt is rcu protected and that member is part of it.
So imo seq_show() is doing that spin_lock unnecessary.