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

[Yonghong Song <yhs@fb.com>]

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.