sys_write() racy for multi-threaded append?

[Michael K. Edwards]

from sys_write():

       file = fget_light(fd, &fput_needed);
       if (file) {
               loff_t pos = file_pos_read(file);
               ret = vfs_write(file, buf, count, &pos);
               file_pos_write(file, pos);
               fput_light(file, fput_needed);
       }

Surely that's racy when two threads write to the same fd and the first
vfs_write call blocks or is preempted.  Or does fget_light take some
per-file lock that I'm not seeing?
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