On Tue 2016-11-29 09:36:00, Paul E. McKenney wrote:
> Updated (but still untested) commit below.
>
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit d3df9bc5fb5d838b049f32a476721eadbc349553
> Author: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Date: Tue Nov 29 05:49:06 2016 -0800
>
> rcu: Once again use NMI-based stack traces in stall warnings
>
> This commit is for all intents and purposes a revert of bc1dce514e9b
> ("rcu: Don't use NMIs to dump other CPUs' stacks"). The reason to suppose
> that this can now safely be reverted is the presence of 42a0bb3f7138
> ("printk/nmi: generic solution for safe printk in NMI"), which is said
> to have made NMI-based stack dumps safe.
>
> However, this reversion keeps one nice property of bc1dce514e9b
> ("rcu: Don't use NMIs to dump other CPUs' stacks"), namely that
> only those CPUs blocking the grace period are dumped. The new
> trigger_single_cpu_backtrace() is used to make this happen, as
> suggested by Josh Poimboeuf.
>
> Reported-by: Vince Weaver <vincent.wea...@maine.edu>
> Not-yet-signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Cc: Petr Mladek <pmla...@suse.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Reviewed-by: Josh Poimboeuf <jpoim...@redhat.com>
Looks fine to me.
Reviewed-by: Petr Mladek <pmla...@suse.com>
Best Regards,
Petr
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 91a68e4e6671..ba0e4825be9d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1396,7 +1396,10 @@ static void rcu_check_gp_kthread_starvation(struct
> rcu_state *rsp)
> }
>
> /*
> - * Dump stacks of all tasks running on stalled CPUs.
> + * Dump stacks of all tasks running on stalled CPUs. First try using
> + * NMIs, but fall back to manual remote stack tracing on architectures
> + * that don't support NMI-based stack dumps. The NMI-triggered stack
> + * traces are more accurate because they are printed by the target CPU.
> */
> static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
> {
> @@ -1406,11 +1409,10 @@ static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
>
> rcu_for_each_leaf_node(rsp, rnp) {
> raw_spin_lock_irqsave_rcu_node(rnp, flags);
> - if (rnp->qsmask != 0) {
> - for_each_leaf_node_possible_cpu(rnp, cpu)
> - if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
> + for_each_leaf_node_possible_cpu(rnp, cpu)
> + if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
> + if (!trigger_single_cpu_backtrace(cpu))
> dump_cpu_task(cpu);
> - }
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> }
> }
>
