Re: Freeing struct lock_file?

From: David Turner <dturner@twopensource.com>
Date: Apr 4, 2015, 00:24 UTC
Message-ID: <1428107083.31333.11.camel@ubuntu>
In-Reply-To: <xmqqfv8gg9bp.fsf@gitster.dls.corp.google.com>

On Fri, 2015-04-03 at 15:01 -0700, Junio C Hamano wrote:

Show 10 quoted lines

> David Turner <dturner@twopensource.com> 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.