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

Yonghong Song
2020-04-10T06:19:31Z

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().

Will use rcu_read_lock() mechanism then.