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

[Z qiang]

>
> 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
> ---
>  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;
>         }
>


Maybe some modifications are missing here:

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index aa4c808978b8..77e7a0dc722a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -867,8 +867,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
        if (time_after(jiffies, rcu_state.jiffies_resched)) {
                if (time_after(jiffies,
                               READ_ONCE(rdp->last_fqs_resched) + jtsq)) {
-                       resched_cpu(rdp->cpu);
                        WRITE_ONCE(rdp->last_fqs_resched, jiffies);
+                       ret = -1;
                }
                if (IS_ENABLED(CONFIG_IRQ_WORK) &&
                    !rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&



Thanks
Zqiang


>         /*
> @@ -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);
>         }
>  }
>