Re: [PATCH AUTOSEL 4.20 028/304] srcu: Prevent __call_srcu() counter wrap with read-side critical section

[Sasha Levin]

On Mon, Jan 28, 2019 at 08:23:18AM -0800, Paul E. McKenney wrote:
> On Mon, Jan 28, 2019 at 10:39:05AM -0500, Sasha Levin wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> >
> > [ Upstream commit 0607ba8403c4cdb253f8c5200ecf654dfb7790cc ]
> >
> > Ever since cdf7abc4610a ("srcu: Allow use of Tiny/Tree SRCU from
> > both process and interrupt context"), it has been permissible
> > to use SRCU read-side critical sections in interrupt context.
> > This allows __call_srcu() to use SRCU read-side critical sections to
> > prevent a new SRCU grace period from ending before the call to either
> > srcu_funnel_gp_start() or srcu_funnel_exp_start completes, thus preventing
> > SRCU grace-period counter overflow during that time.
> >
> > Note that this does not permit removal of the counter-wrap checks in
> > srcu_gp_end().  These check are necessary to handle the case where
> > a given CPU does not interact at all with SRCU for an extended time
> > period.
> >
> > This commit therefore adds an SRCU read-side critical section to
> > __call_srcu() in order to prevent grace period counter wrap during
> > the funnel-locking process.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
>
> Hello, Sasha,
>
> I recommend -against- backporting this one.  It is a theoretical bug
> that requires rather long preemption, so the risk of backporting likely
> greatly exceeeds the risk of the bug actually happening, especially on
> 64-bit systems.

I'll drop it, thanks Paul!

--
Thanks,
Sasha