Re: [PATCH] rcu: Break rcu_node_0 --> &rq->__lock order

[Waiman Long]

On 10/31/23 16:02, Peter Zijlstra wrote:
> On Tue, Oct 31, 2023 at 07:29:04AM -0700, Paul E. McKenney wrote:
> > Other than the de-alphabetization of the local variables, it looks
> > plausible to me.  Frederic's suggestion also sounds plausible to me.
> Having spend the better part of the past two decades using upside down
> xmas trees for local variables, this alphabet thing is obnoxious :-)
>
> But your code, your rules.
>
> To reduce the number of alphabet songs required, I've taken the liberty
> to move a few variables into a narrower scope, hope that doesn't offend.
>
> ---
> Subject: rcu: Break rcu_node_0 --> &rq->__lock order
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Tue, 31 Oct 2023 09:53:08 +0100
>
> Commit 851a723e45d1 ("sched: Always clear user_cpus_ptr in
> do_set_cpus_allowed()") added a kfree() call to free any user
> provided affinity mask, if present. It was changed later to use
> kfree_rcu() in commit 9a5418bc48ba ("sched/core: Use kfree_rcu()
> in do_set_cpus_allowed()") to avoid a circular locking dependency
> problem.
>
> It turns out that even kfree_rcu() isn't safe for avoiding
> circular locking problem. As reported by kernel test robot,
> the following circular locking dependency now exists:
>
>    &rdp->nocb_lock --> rcu_node_0 --> &rq->__lock
>
> Solve this by breaking the rcu_node_0 --> &rq->__lock chain by moving
> the resched_cpu() out from under rcu_node lock.
>
> [peterz: heavily borrowed from Waiman's Changelog]
> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/oe-lkp/202310302207.a25f1a30-oliver.sang@xxxxxxxxx

Thanks for addressing this issue. I am fine with your way as long as it 
gets the job done. I am not familiar enough of the RCU code to do a 
proper review, but I do get the general idea of your change and it looks 
good to me.

Acked-by: Waiman Long <longman@xxxxxxxxxx>

> ---
>   kernel/rcu/tree.c |   34 ++++++++++++++++++++++++----------
>   1 file changed, 24 insertions(+), 10 deletions(-)
>
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -754,14 +754,19 @@ static int dyntick_save_progress_counter
>   }
>   
>   /*
> - * Return true if the specified CPU has passed through a quiescent
> - * state by virtue of being in or having passed through an dynticks
> - * idle state since the last call to dyntick_save_progress_counter()
> - * for this same CPU, or by virtue of having been offline.
> + * Returns positive if the specified CPU has passed through a quiescent state
> + * by virtue of being in or having passed through an dynticks idle state since
> + * the last call to dyntick_save_progress_counter() for this same CPU, or by
> + * virtue of having been offline.
> + *
> + * Returns negative if the specified CPU needs a force resched.
> + *
> + * Returns zero otherwise.
>    */
>   static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
>   {
>   	unsigned long jtsq;
> +	int ret = 0;
>   	struct rcu_node *rnp = rdp->mynode;
>   
>   	/*
> @@ -847,8 +852,8 @@ static int rcu_implicit_dynticks_qs(stru
>   	    (time_after(jiffies, READ_ONCE(rdp->last_fqs_resched) + jtsq * 3) ||
>   	     rcu_state.cbovld)) {
>   		WRITE_ONCE(rdp->rcu_urgent_qs, true);
> -		resched_cpu(rdp->cpu);
>   		WRITE_ONCE(rdp->last_fqs_resched, jiffies);
> +		ret = -1;
>   	}
>   
>   	/*
> @@ -891,7 +896,7 @@ static int rcu_implicit_dynticks_qs(stru
>   		}
>   	}
>   
> -	return 0;
> +	return ret;
>   }
>   
>   /* Trace-event wrapper function for trace_rcu_future_grace_period.  */
> @@ -2257,15 +2262,15 @@ static void force_qs_rnp(int (*f)(struct
>   {
>   	int cpu;
>   	unsigned long flags;
> -	unsigned long mask;
> -	struct rcu_data *rdp;
>   	struct rcu_node *rnp;
>   
>   	rcu_state.cbovld = rcu_state.cbovldnext;
>   	rcu_state.cbovldnext = false;
>   	rcu_for_each_leaf_node(rnp) {
> +		unsigned long mask = 0;
> +		unsigned long rsmask = 0;
> +
>   		cond_resched_tasks_rcu_qs();
> -		mask = 0;
>   		raw_spin_lock_irqsave_rcu_node(rnp, flags);
>   		rcu_state.cbovldnext |= !!rnp->cbovldmask;
>   		if (rnp->qsmask == 0) {
> @@ -2283,11 +2288,17 @@ static void force_qs_rnp(int (*f)(struct
>   			continue;
>   		}
>   		for_each_leaf_node_cpu_mask(rnp, cpu, rnp->qsmask) {
> +			struct rcu_data *rdp;
> +			int ret;
> +
>   			rdp = per_cpu_ptr(&rcu_data, cpu);
> -			if (f(rdp)) {
> +			ret = f(rdp);
> +			if (ret > 0) {
>   				mask |= rdp->grpmask;
>   				rcu_disable_urgency_upon_qs(rdp);
>   			}
> +			if (ret < 0)
> +				rsmask |= rdp->grpmask;
>   		}
>   		if (mask != 0) {
>   			/* Idle/offline CPUs, report (releases rnp->lock). */
> @@ -2296,6 +2307,9 @@ static void force_qs_rnp(int (*f)(struct
>   			/* Nothing to do here, so just drop the lock. */
>   			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>   		}
> +
> +		for_each_leaf_node_cpu_mask(rnp, cpu, rsmask)
> +			resched_cpu(cpu);
>   	}
>   }
>