Re: [PATCH] sched: Don't call any kfree*() API in do_set_cpus_allowed()

[Paul E. McKenney]

On Tue, Oct 31, 2023 at 09:53:08AM +0100, Peter Zijlstra wrote:
> On Mon, Oct 30, 2023 at 08:14:18PM -0400, Waiman Long wrote:
> > 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 still exists:
> > 
> >   &rdp->nocb_lock --> rcu_node_0 --> &rq->__lock
> > 
> > So no kfree*() API can be used in do_set_cpus_allowed(). To prevent
> > memory leakage, the unused user provided affinity mask is now saved in a
> > lockless list to be reused later by subsequent sched_setaffinity() calls.
> > 
> > Without kfree_rcu(), the internal cpumask_rcuhead union can be removed
> > too as a lockless list entry only holds a single pointer.
> > 
> > Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
> 
> Bah, or we fix RCU...  Paul, how insane is the below?

Other than the de-alphabetization of the local variables, it looks
plausible to me.  Frederic's suggestion also sounds plausible to me.

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 31 +++++++++++++++++++++----------
>  1 file changed, 21 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index cb1caefa8bd0..4b8e26a028ee 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -754,15 +754,20 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp)
>  }
>  
>  /*
> - * 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;
>  	struct rcu_node *rnp = rdp->mynode;
> +	unsigned long jtsq;
> +	int ret = 0;
>  
>  	/*
>  	 * If the CPU passed through or entered a dynticks idle phase with
> @@ -847,8 +852,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
>  	    (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(struct rcu_data *rdp)
>  		}
>  	}
>  
> -	return 0;
> +	return ret;
>  }
>  
>  /* Trace-event wrapper function for trace_rcu_future_grace_period.  */
> @@ -2255,11 +2260,11 @@ void rcu_sched_clock_irq(int user)
>   */
>  static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
>  {
> -	int cpu;
> +	unsigned long mask, rsmask = 0;
>  	unsigned long flags;
> -	unsigned long mask;
>  	struct rcu_data *rdp;
>  	struct rcu_node *rnp;
> +	int cpu, ret;
>  
>  	rcu_state.cbovld = rcu_state.cbovldnext;
>  	rcu_state.cbovldnext = false;
> @@ -2284,10 +2289,13 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
>  		}
>  		for_each_leaf_node_cpu_mask(rnp, cpu, rnp->qsmask) {
>  			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 |= 1UL << (cpu - rnp->grplo);
>  		}
>  		if (mask != 0) {
>  			/* Idle/offline CPUs, report (releases rnp->lock). */
> @@ -2296,6 +2304,9 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
>  			/* 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);
>  	}
>  }
>