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.
