Subject: Re: Freeing struct lock_file?

From: David Turner <[email-masked]>
Date: Fri, 03 Apr 2015 20:24:43 -0400

On Fri, 2015-04-03 at 15:01 -0700, Junio C Hamano wrote:
> David Turner <[email-masked]> writes:
> 
> > Why is it impossible to free struct lock_files?  I understand that they
> > become part of a linked list, and that there's an atexit handler that
> > goes over that list.  But couldn't we just remove them from the linked
> > list and then free them? 
> 
> I suspect that the code is worried about getting a signal, while it
> is manipulating the linked list, and then cause the atexit handler
> to walk a list that is in a broken state.

This is technically possible, but practically unlikely: aligned
pointer-sized writes are atomic on very nearly every processor, and that
is all that is required to remove an item from a linked list safely in
this case (though not, of course, in the general multi-threaded case).

But I can see why git wouldn't want to depend on that behavior. C11 has
a way to do this safely, but AIUI, git doesn't want to move to C99 let
alone C11.  So I guess this will just have to remain the way it is.