Re: [PATCH] rcu/nocb: Add Safe checks for access offloaded rdp

[Z qiang <qiang.zhang1211@gmail.com>]

>
> Le Mon, Apr 28, 2025 at 05:54:03PM +0800, Zqiang a écrit :
> > For Preempt-RT kernel, when enable CONFIG_PROVE_RCU Kconfig,
> > disable local bh in rcuc kthreads will not affect preempt_count(),
> > this resulted in the following splat:
> >
> > WARNING: suspicious RCU usage
> > kernel/rcu/tree_plugin.h:36 Unsafe read of RCU_NOCB offloaded state!
> > stack backtrace:
> > CPU: 0 UID: 0 PID: 22 Comm: rcuc/0
> > Call Trace:
> > [    0.407907]  <TASK>
> > [    0.407910]  dump_stack_lvl+0xbb/0xd0
> > [    0.407917]  dump_stack+0x14/0x20
> > [    0.407920]  lockdep_rcu_suspicious+0x133/0x210
> > [    0.407932]  rcu_rdp_is_offloaded+0x1c3/0x270
> > [    0.407939]  rcu_core+0x471/0x900
> > [    0.407942]  ? lockdep_hardirqs_on+0xd5/0x160
> > [    0.407954]  rcu_cpu_kthread+0x25f/0x870
> > [    0.407959]  ? __pfx_rcu_cpu_kthread+0x10/0x10
> > [    0.407966]  smpboot_thread_fn+0x34c/0xa50
> > [    0.407970]  ? trace_preempt_on+0x54/0x120
> > [    0.407977]  ? __pfx_smpboot_thread_fn+0x10/0x10
> > [    0.407982]  kthread+0x40e/0x840
> > [    0.407990]  ? __pfx_kthread+0x10/0x10
> > [    0.407994]  ? rt_spin_unlock+0x4e/0xb0
> > [    0.407997]  ? rt_spin_unlock+0x4e/0xb0
> > [    0.408000]  ? __pfx_kthread+0x10/0x10
> > [    0.408006]  ? __pfx_kthread+0x10/0x10
> > [    0.408011]  ret_from_fork+0x40/0x70
> > [    0.408013]  ? __pfx_kthread+0x10/0x10
> > [    0.408018]  ret_from_fork_asm+0x1a/0x30
> > [    0.408042]  </TASK>
> >
> > Currently, triggering an rdp offloaded state change need the
> > corresponding rdp's CPU goes offline, and at this time the rcuc
> > kthreads has already in parking state. this means the corresponding
> > rcuc kthreads can safely read offloaded state of rdp while it's
> > corresponding cpu is online.
> >
> > This commit therefore add rdp->rcu_cpu_kthread_task check for
> > Preempt-RT kernels.
> >
> > Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
> > ---
> >  kernel/rcu/tree_plugin.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 003e549f6514..fe728eded36e 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -31,7 +31,9 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
> >                 lockdep_is_held(&rcu_state.nocb_mutex) ||
> >                 (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) &&
> >                  rdp == this_cpu_ptr(&rcu_data)) ||
> > -               rcu_current_is_nocb_kthread(rdp)),
> > +               rcu_current_is_nocb_kthread(rdp) ||
> > +               (IS_ENABLED(CONFIG_PREEMPT_RT) &&
> > +                current == rdp->rcu_cpu_kthread_task)),
>
> Isn't it safe also on !CONFIG_PREEMPT_RT ?

For !CONFIG_PREEMPT_RT and  in rcuc kthreads, it's also safe,
but the following check will passed :

(!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) &&
          rdp == this_cpu_ptr(&rcu_data))

Thanks
Zqiang


>
> Thanks.
>
> >               "Unsafe read of RCU_NOCB offloaded state"
> >       );
> >
> > --
> > 2.17.1
> >
>
> --
> Frederic Weisbecker
> SUSE Labs