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

[Paul E. McKenney]

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()?

							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
> > > 
> > 
> 
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