svn commit: r334708 - head/sys/kern

[Mark Johnston]

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.