Re: [PATCHv2 2/2] srcu: Eliminate the case that snp_seq bigger than snap in srcu_funnel_gp_start()

[Pingfan Liu <kernelfans@gmail.com>]

On Thu, Nov 24, 2022 at 08:19:26AM -0800, Paul E. McKenney wrote:
> On Wed, Nov 23, 2022 at 09:24:17PM -0800, Paul E. McKenney wrote:
> > On Thu, Nov 24, 2022 at 12:06:39PM +0800, Pingfan Liu wrote:
> > > On Thu, Nov 24, 2022 at 11:33 AM Pingfan Liu <kernelfans@gmail.com> wrote:
> > > >
> > > > On Wed, Nov 23, 2022 at 11:20:46AM -0800, Paul E. McKenney wrote:
> > > > > On Wed, Nov 23, 2022 at 09:56:38PM +0800, Pingfan Liu wrote:
> > > > > > Since the srcu read lock is still held during srcu_funnel_gp_start(),
> > > > > > the seq snap should be the largest number for the slot
> > > > > > srcu_have_cbs[idx].
> > > > > >
> > > > > > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > > > > > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > > > > > Cc: "Paul E. McKenney" <paulmck@kernel.org>
> > > > > > Cc: Frederic Weisbecker <frederic@kernel.org>
> > > > > > Cc: Josh Triplett <josh@joshtriplett.org>
> > > > > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > > > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > > > > To: rcu@vger.kernel.org
> > > > > > ---
> > > > > >  include/linux/rcupdate.h |  1 +
> > > > > >  kernel/rcu/srcutree.c    | 11 ++++++-----
> > > > > >  2 files changed, 7 insertions(+), 5 deletions(-)
> > > > > >
> > > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > > > > index 08605ce7379d..a09007236660 100644
> > > > > > --- a/include/linux/rcupdate.h
> > > > > > +++ b/include/linux/rcupdate.h
> > > > > > @@ -32,6 +32,7 @@
> > > > > >  #include <linux/context_tracking_irq.h>
> > > > > >
> > > > > >  #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> > > > > > +#define ULONG_CMP_GT(a, b) (ULONG_MAX / 2 > (a) - (b))
> > > > >
> > > > > Please see below...
> > > > >
> > > > > >  #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> > > > > >  #define ulong2long(a)              (*(long *)(&(a)))
> > > > > >  #define USHORT_CMP_GE(a, b)        (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> > > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > > > > index 2fc0e775ade4..41902b823687 100644
> > > > > > --- a/kernel/rcu/srcutree.c
> > > > > > +++ b/kernel/rcu/srcutree.c
> > > > > > @@ -905,14 +905,15 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> > > > > >             for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
> > > > > >                     spin_lock_irqsave_rcu_node(snp, flags);
> > > > > >                     snp_seq = snp->srcu_have_cbs[idx];
> > > > > > -                   if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) {
> > > > > > +                   /*
> > > > > > +                    * s should be the biggest in the current slot. Hence only LE is
> > > > > > +                    * valid
> > > > > > +                    */
> > > > > > +                   BUG_ON(ULONG_CMP_GT(snp_seq, s));
> > > > >
> > > > > Why not this?  (Plus adjusting the comment above, of course.)
> > > > >
> > > > >                       WARN_ON_ONCE(ULONG_CMP_LT(s, snp_seq));
> > > > >
> > > >
> > > > A neat solution!
> > > >
> > > > And what about the comment
> > > >                         /* s should be the biggest in the current slot. */
> > > >
> > > > > That way we don't need ULONG_CMP_GT().  Plus if we are confused about
> > > > > s being the biggest in the current slot, we get a splat and can debug
> > > > > further.  We both might be quite sure that we are not confused, but
> > > > > that is exactly when we are most prone to making mistakes.  ;-)
> > > > >
> > > >
> > > > Same feeling :)
> > > >
> > > > > I also ask that you run with this check for some time.  After all, if
> > > 
> > > Forget to ask if  the test
> > >   "tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration
> > > 10h --configs 18*SRCU-P"
> > > satisfies your requirement?
> > 
> > I am thinking more in terms of adding the WARN_ON_ONCE(), letting it be
> > in -next and possibly mainline for a couple of years, and then if there
> > are no splats, start feeling more confident in the asserted relationship.
> > 
> > If there was a significant performance, scalability, energy-efficiency, or
> > simplification benefit, I would feel justified in being more aggressive.
> > 
> > But I am not seeing a significant benefit.
> 
> Ah, and before I forget -again-, have you thought through the counter-wrap
> scenarios?  Please keep in mind that on a 32-bit system, those counters
> can wrap quite quickly compared to typical uptimes.
> 

I think 32-bit has not significant different since the statement
  WARN_ON_ONCE(ULONG_CMP_LT(s, snp_seq));

where 's < snp_seq' is not associated with bits. Am I missing anything?

Thanks,

	Pingfan

> 							Thanx, Paul
> 
> > > Thanks
> > > 
> > >     Pingfan
> > > > Sure. I will try it immediately.
> > > >
> > > > > the assumption is incorrect, the resulting SRCU hangs off in various
> > > > > systems around the world will not be so much fun to debug.
> > > > >
> > > > > Given that this is slowpath code, it is much better to take an extra
> > > > > compare and branch than to introduce even an extremely small risk of
> > > > > hanging SRCU.
> > > > >
> > > >
> > > > Agree.
> > > >
> > > >
> > > > Thanks,
> > > >
> > > >         Pingfan
> > > >
> > > > >                                                       Thanx, Paul
> > > > >
> > > > > > +                   if (!srcu_invl_snp_seq(snp_seq) && (snp_seq == s)) {
> > > > > >                             if (snp == snp_leaf && snp_seq == s)
> > > > > >                                     snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
> > > > > >                             spin_unlock_irqrestore_rcu_node(snp, flags);
> > > > > > -                           if (snp == snp_leaf && snp_seq != s) {
> > > > > > -                                   srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0);
> > > > > > -                                   return;
> > > > > > -                           }
> > > > > >                             if (!do_norm)
> > > > > >                                     srcu_funnel_exp_start(ssp, snp, s);
> > > > > >                             return;
> > > > > > --
> > > > > > 2.31.1
> > > > > >