How to modify write permission on current buffer in emacs?

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http://unix.stackexchange.com – Is it possible to change the write permissions on a file from inside emacs, without killing/re-opening the buffer? Sometimes I forget to modify the permissions on a file before opening it. I can modify the permissions from inside emacs (M-! chmod u+w filename) but this doesn't update the buffer which remains write protected and refuses to modify the file. Is there a way to update permissions inside the buffer? Bonus point if I can assign this to a shortcut! (HowTos)