On Thu, Sep 10, 2026 at 8:13 AM Paul E. McKenney <[email protected]> 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" <[email protected]>
> > wrote:
> >
> > > On Mon, Sep 07, 2026 at 03:58:23PM +0800, Kunwu Chan wrote:
> > > > From: Kunwu Chan <[email protected]>
> > > >
> > > > 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 <[email protected]>
> > >
> > > 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.
So I agree the srcu_gp_end() patch can be dropped.
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)
