Re: [PATCH 07/13] srcutree: Don't transition atomic SRCU to big in srcu_gp_end()

[KunWu Chan]

On Thu, Sep 10, 2026 at 11:46 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Thu, Sep 10, 2026 at 11:18:20AM +0800, KunWu Chan wrote:
> > On Thu, Sep 10, 2026 at 8:13 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Sep 09, 2026 at 10:45:14AM +0800, Kunwu Chan wrote:
> > > > On Tue, 8 Sep 2026 16:38:19 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> > > >
> > > > > On Mon, Sep 07, 2026 at 03:58:23PM +0800, Kunwu Chan wrote:
> > > > > > From: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> > > > > >
> > > > > > The transition-to-big code path in srcu_gp_end() calls
> > > > > > init_srcu_struct_nodes() with GFP_KERNEL, which is illegal in the
> > > > > > atomic context reachable from synchronize_srcu_atomic().  Atomic SRCU
> > > > > > has no use for the srcu_node combining tree: it has neither callbacks
> > > > > > nor srcu_barrier() operations, and its grace periods are serialized
> > > > > > by ->srcu_atomic_gp_flag instead.
> > > > > >
> > > > > > Skip this transition entirely for atomic SRCU, which is also defense
> > > > > > in depth against any path that might wrongly set ->srcu_size_state for
> > > > > > an atomic srcu_struct.
> > > > > >
> > > > > > Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> > > > >
> > > > > Again, given that we have this commit, is this patch needed?
> > > > >
> > > > > 4e01d5320a2f ("srcutree: Suppress to-big transition for atomic SRCU")
> > > > >
> > > > > And again, if so, please tell me what I am missing.
> > > >
> > > > Yes, this is a separate transition path. That commit suppresses the
> > > > initialization-time transition, while this patch prevents the transition
> > > > from srcu_gp_end(), which can otherwise call
> > > > init_srcu_struct_nodes(..., GFP_KERNEL) for an atomic SRCU.
> > > >
> > > > So this is defense in depth for the atomic path.
> > >
> > > Fair point.  But is there any additional pathway other than the
> > > SRCU_SIZING_IS_TORTURE() check at the end of srcu_torture_stats_print()?
> >
> > I checked all transition-to-big pathways for atomic SRCU.
> > Atomic SRCU can reach srcu_gp_end(), but its srcu_size_state remains
> > SRCU_SIZE_SMALL,
> > so the transition condition there cannot be satisfied.
> >
> > The contention path is also unreachable:
> > synchronize_srcu_atomic() calls srcu_gp_start() directly and never
> > goes through srcu_gp_start_if_needed().
> > The other callers of that path reject atomic SRCU.
> >
> > The only remaining explicit transition is SRCU_SIZING_IS_TORTURE() in
> > srcu_torture_stats_print(),
> > which currently lacks an atomic exclusion. I'll send a follow-up patch
> > to add a srcu_reader_flavor check there.
>
> Very good!
>
> > So I agree the srcu_gp_end() patch can be dropped.
>
> It might not hurt to have a WARN_ON_ONCE() there, just in case someone
> implements another pathway.

Thanks, Paul. I'll send a v2 of this patch separately, dropping
this patch change and adding the WARN_ON_ONCE() before the
transition check:
+ WARN_ON_ONCE(ssp->srcu_reader_flavor == SRCU_READ_FLAVOR_ATOMIC &&
    ss_state != SRCU_SIZE_SMALL);

if (ss_state != SRCU_SIZE_SMALL &&
   ss_state != SRCU_SIZE_BIG) { ... ....

Thanks,
Kunwu

>
>                                                         Thanx, Paul
>
> > Thanks,
> > Kunwu
> >
> > >
> > >                                                         Thanx, Paul
> > >
> > > > Thanks,
> > > > KunWu
> > > >
> > > > >
> > > > >                                                     Thanx, Paul
> > > > >
> > > > > > ---
> > > > > >  kernel/rcu/srcutree.c | 5 +++--
> > > > > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > > > > index 01f224a56b41..74acd5645d4c 100644
> > > > > > --- a/kernel/rcu/srcutree.c
> > > > > > +++ b/kernel/rcu/srcutree.c
> > > > > > @@ -1074,8 +1074,9 @@ static void srcu_gp_end(struct srcu_struct *ssp, bool is_atomic)
> > > > > >           raw_spin_unlock_irq_rcu_node(sup);
> > > > > >   }
> > > > > >
> > > > > > - /* Transition to big if needed. */
> > > > > > - if (ss_state != SRCU_SIZE_SMALL && ss_state != SRCU_SIZE_BIG) {
> > > > > > + /* Transition to big if needed, but never for atomic SRCU. */
> > > > > > + if (!is_atomic && ss_state != SRCU_SIZE_SMALL &&
> > > > > > +     ss_state != SRCU_SIZE_BIG) {
> > > > > >           if (ss_state == SRCU_SIZE_ALLOC)
> > > > > >                   init_srcu_struct_nodes(ssp, GFP_KERNEL);
> > > > > >           else
> > > > > > --
> > > > > > 2.43.0
> > > > > >
> > > > >
> > > >
> > > > Sent using hkml (https://github.com/sjp38/hackermail)