Re: racy acccess in kern_runq.c

[Maxime Villard <max%m00nbsd.net@localhost>]

Le 06/12/2019 à 20:37, Mindaugas Rasiukevicius a écrit :
> Maxime Villard <max%m00nbsd.net@localhost> wrote:
> > Le 06/12/2019 à 17:53, Andrew Doran a écrit :
> > > On Fri, Dec 06, 2019 at 05:22:39PM +0900, Kengo NAKAHARA wrote:
> > >
> > > >    	/* Update the worker */
> > > > -	worker_ci = hci;
> > > > +	atomic_swap_ptr(&worker_ci, hci);
> > >
> > > Why atomic_swap_ptr() not atomic_store_relaxed()?  I don't see any bug
> > > that it fixes.  Other than that it look OK to me.
> >
> > Because I suggested it; my concern was that if not explicitly atomic, the
> > cpu could make two writes internally (even though the compiler produces
> > only one instruction), and in that case a page fault would have been
> > possible because of garbage dereference.
>
> No, atomic_store_relaxed() is sufficient; that is what it is for.

Alright.

The patch needs to be completed though; ie, 'spc_mcount++' too should use
relaxed atomics, UVM style ([1]).

[1] https://nxr.netbsd.org/diff/src/sys/uvm/uvm_fault.c?r2=%2Fsrc%2Fsys%2Fuvm%2Fuvm_fault.c%401.209&r1=%2Fsrc%2Fsys%2Fuvm%2Fuvm_fault.c%401.208