svn commit: r334708 - head/sys/kern

[Mark Johnston]

On Thu, Jul 19, 2018 at 12:09:21PM -0700, Bryan Drewery wrote:
> Did this issue get resolved?

It's fixed by r337974.

> On 6/8/2018 11:37 AM, Konstantin Belousov wrote:
> > On Fri, Jun 08, 2018 at 02:30:10PM -0400, Mark Johnston wrote:
> >> On Fri, Jun 08, 2018 at 08:37:55PM +0300, Konstantin Belousov wrote:
> >>> On Thu, Jun 07, 2018 at 11:02:29PM -0700, Ryan Libby wrote:
> >>>> On Thu, Jun 7, 2018 at 10:03 PM, Mateusz Guzik <mjguzik at gmail.com> wrote:
> >>>>> Checking it without any locks is perfectly valid in this case. It is done
> >>>>> after v_holdcnt gets bumped from a non-zero value. So at that time it
> >>>>> is at least two. Of course that result is stale as an arbitrary number of
> >>>>> other threads could have bumped and dropped the ref past that point.
> >>>>> The minimum value is 1 since we hold the ref. But this means the
> >>>>> vnode must not be on the free list and that's what the assertion is
> >>>>> verifying.
> >>>>>
> >>>>> The problem is indeed lack of ordering against the code clearing the
> >>>>> flag for the case where 2 threads to vhold and one does the 0->1
> >>>>> transition.
> >>>>>
> >>>>> That said, the fence is required for the assertion to work.
> >>>>>
> >>>>
> >>>> Yeah, I agree with this logic.  What I mean is that reordering between
> >>>> v_holdcnt 0->1 and v_iflag is normally settled by the release and
> >>>> acquisition of the vnode interlock, which we are supposed to hold for
> >>>> v_*i*flag.  A quick scan seems to show all of the checks of VI_FREE that
> >>>> are not asserts do hold the vnode interlock.  So, I'm just saying that I
> >>>> don't think the possible reordering affects them.
> >>> But do we know that only VI_FREE checks are affected ?
> >>>
> >>> My concern is that users of _vhold() rely on seeing up to date state of the
> >>> vnode, and VI_FREE is only an example of the problem.  Most likely, the
> >>> code which fetched the vnode pointer before _vhold() call, should guarantee
> >>> visibility.
> >>
> >> Wouldn't this be a problem only if we permit lockless accesses of vnode
> >> state outside of _vhold() and other vnode subroutines? The current
> >> protocol requires that the interlock be held, and this synchronizes with
> >> code which performs 0->1 and 1->0 transitions of the hold count. If this
> >> requirement is relaxed in the future, then fences would indeed be
> >> needed.
> > 
> > I do not claim that my concern is a real problem. I stated it as a
> > thing to look at when deciding whether the fences should be added
> > (unconditionally ?).
> > 
> > If you argument is that the only current lock-less protocol for the
> > struct vnode state is the v_holdcnt transitions for > 1, then I can
> > agree with it.
> > 
> 
> 
> -- 
> Regards,
> Bryan Drewery
>