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

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Message-ID: <aA9U9QvB2t2MLuU2@pavilion.home>
Date: Mon, 28 Apr 2025 12:14:13 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Zqiang <qiang.zhang1211@...il.com>
Cc: 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

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 ?

Thanks.

>  		"Unsafe read of RCU_NOCB offloaded state"
>  	);
>  
> -- 
> 2.17.1
> 

-- 
Frederic Weisbecker
SUSE Labs