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

[Kunwu Chan]

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.

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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