linux-kernel - Re: [PATCH] rcu: Fix suspicious RCU usage in __do

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Message-ID: <9f227fd2-13b2-4c78-b3b3-05ef6d583078@paulmck-laptop>
Date: Mon, 29 Apr 2024 13:45:44 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Zqiang <qiang.zhang1211@...il.com>
Cc: tglx@...utronix.de, frederic@...nel.org, neeraj.upadhyay@...nel.org,
	joel@...lfernandes.org, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Fix suspicious RCU usage in __do_softirq()

On Sat, Apr 27, 2024 at 06:28:08PM +0800, Zqiang wrote:
> Currently, the condition "__this_cpu_read(ksoftirqd) == current" is
> checked to ensure the rcu_softirq_qs() is invoked in ksoftirqd tasks
> context for non-RT kernels. however, in some scenarios, this condition
> will be broken.
> 
>      ksoftirqd/0
> ->finish_task_switch
>   ->put_task_struct_rcu_user
>     ->call_rcu(&task->rcu, delayed_put_task_struct)
>       ->__kasan_record_aux_stack
>         ->pfn_valid
>           ->rcu_read_lock_sched()
>             <interrupt>
>              __irq_exit_rcu
>              ->__do_softirq
>                -> if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
>                    __this_cpu_read(ksoftirqd) == current)
>                    ->rcu_softirq_qs
>                      ->RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map))
> 
> The rcu quiescent states is reported occurs in the rcu-read critical
> section, so the lockdep warning is triggered. this commit therefore
> remove "__this_cpu_read(ksoftirqd) == current" condition check, generate
> new "handle_softirqs(bool kirqd)" function to replace __do_softirq() in
> run_ksoftirqdt(), and set parameter kirqd to true, make rcu_softirq_qs()
> be invoked only in ksofirqd tasks context for non-RT kernels.
> 
> Reported-by: syzbot+dce04ed6d1438ad69656@...kaller.appspotmail.com
> Link: https://lore.kernel.org/lkml/8f281a10-b85a-4586-9586-5bbc12dc784f@paulmck-laptop/T/#mea8aba4abfcb97bbf499d169ce7f30c4cff1b0e3
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Zqiang <qiang.zhang1211@...il.com>

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/softirq.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index b315b21fb28c..e991d735be0d 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -508,7 +508,7 @@ static inline bool lockdep_softirq_start(void) { return false; }
>  static inline void lockdep_softirq_end(bool in_hardirq) { }
>  #endif
>  
> -asmlinkage __visible void __softirq_entry __do_softirq(void)
> +static void handle_softirqs(bool kirqd)
>  {
>  	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
>  	unsigned long old_flags = current->flags;
> @@ -563,8 +563,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>  		pending >>= softirq_bit;
>  	}
>  
> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
> -	    __this_cpu_read(ksoftirqd) == current)
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kirqd)
>  		rcu_softirq_qs();
>  
>  	local_irq_disable();
> @@ -584,6 +583,11 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>  	current_restore_flags(old_flags, PF_MEMALLOC);
>  }
>  
> +asmlinkage __visible void __softirq_entry __do_softirq(void)
> +{
> +	handle_softirqs(false);
> +}
> +
>  /**
>   * irq_enter_rcu - Enter an interrupt context with RCU watching
>   */
> @@ -921,7 +925,7 @@ static void run_ksoftirqd(unsigned int cpu)
>  		 * We can safely run softirq on inline stack, as we are not deep
>  		 * in the task stack here.
>  		 */
> -		__do_softirq();
> +		handle_softirqs(true);
>  		ksoftirqd_run_end();
>  		cond_resched();
>  		return;
> -- 
> 2.17.1
>