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

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Message-ID: <CALm+0cUx2siBvaRYwWGsN21PC=UUUy1EqLTRe5HmRW2ECWtUWA@mail.gmail.com>
Date: Wed, 30 Apr 2025 22:57:59 +0800
From: Z qiang <qiang.zhang1211@...il.com>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Frederic Weisbecker <frederic@...nel.org>, paulmck@...nel.org, neeraj.upadhyay@...nel.org, 
	joel@...lfernandes.org, urezki@...il.com, boqun.feng@...il.com, 
	rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/nocb: Add Safe checks for access offloaded rdp

>
>
>
> On 4/28/2025 6:59 AM, Z qiang wrote:
> >>
> >> 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@...il.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))
>
> I think the fact that it already passes for !PREEMPT_RT does not matter, because
> it simplifies the code so drop the PREEMPT_RT check?
>
> Or will softirq_count() not work? It appears to have special casing for
> PREEMPT_RT's local_bh_disable():
>
> (   ( !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) || softirq_count() )
>    && rdp == this_cpu_ptr(&rcu_data))  )

Thank you for Joel's reply,  I also willing to accept such
modifications and resend :) .

Thanks
Zqiang
>
> thanks,
>
> - Joel
>
>
>
>
>