Hi all,

Today's linux-next merge of the paulmck tree got a conflict in:

  kernel/rcu/tree_exp.h

between commit:

  9a30ceb4d93e ("rcu: Mark emergency sections in rcu stalls")

from the printk tree and commit:

  c19a6725b019 ("rcu: Extract synchronize_rcu_expedited_stall() from synchronize_rcu_expedited_wait()")

from the paulmck tree.

I fixed it up (see below) and can carry the fix as necessary. This
is now fixed as far as linux-next is concerned, but any non trivial
conflicts should be mentioned to your upstream maintainer when your tree
is submitted for merging.  You may also want to consider cooperating
with the maintainer of the conflicting tree to minimise any particularly
complex conflicts.

-- 
Cheers,
Stephen Rothwell

diff --cc kernel/rcu/tree_exp.h
index be2d251e84f8,d4be644afb50..000000000000
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@@ -543,6 -542,69 +543,70 @@@ static bool synchronize_rcu_expedited_w
  	return false;
  }
  
+ /*
+  * Print out an expedited RCU CPU stall warning message.
+  */
+ static void synchronize_rcu_expedited_stall(unsigned long jiffies_start, unsigned long j)
+ {
+ 	int cpu;
+ 	unsigned long mask;
+ 	int ndetected;
+ 	struct rcu_node *rnp;
+ 	struct rcu_node *rnp_root = rcu_get_root();
+ 
+ 	if (READ_ONCE(csd_lock_suppress_rcu_stall) && csd_lock_is_stuck()) {
+ 		pr_err("INFO: %s detected expedited stalls, but suppressed full report due to a stuck CSD-lock.\n", rcu_state.name);
+ 		return;
+ 	}
+ 	pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {", rcu_state.name);
+ 	ndetected = 0;
+ 	rcu_for_each_leaf_node(rnp) {
+ 		ndetected += rcu_print_task_exp_stall(rnp);
+ 		for_each_leaf_node_possible_cpu(rnp, cpu) {
+ 			struct rcu_data *rdp;
+ 
+ 			mask = leaf_node_cpu_bit(rnp, cpu);
+ 			if (!(READ_ONCE(rnp->expmask) & mask))
+ 				continue;
+ 			ndetected++;
+ 			rdp = per_cpu_ptr(&rcu_data, cpu);
+ 			pr_cont(" %d-%c%c%c%c", cpu,
+ 				"O."[!!cpu_online(cpu)],
+ 				"o."[!!(rdp->grpmask & rnp->expmaskinit)],
+ 				"N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
+ 				"D."[!!data_race(rdp->cpu_no_qs.b.exp)]);
+ 		}
+ 	}
+ 	pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
+ 		j - jiffies_start, rcu_state.expedited_sequence, data_race(rnp_root->expmask),
+ 		".T"[!!data_race(rnp_root->exp_tasks)]);
+ 	if (ndetected) {
+ 		pr_err("blocking rcu_node structures (internal RCU debug):");
+ 		rcu_for_each_node_breadth_first(rnp) {
+ 			if (rnp == rnp_root)
+ 				continue; /* printed unconditionally */
+ 			if (sync_rcu_exp_done_unlocked(rnp))
+ 				continue;
+ 			pr_cont(" l=%u:%d-%d:%#lx/%c",
+ 				rnp->level, rnp->grplo, rnp->grphi, data_race(rnp->expmask),
+ 				".T"[!!data_race(rnp->exp_tasks)]);
+ 		}
+ 		pr_cont("\n");
+ 	}
+ 	rcu_for_each_leaf_node(rnp) {
+ 		for_each_leaf_node_possible_cpu(rnp, cpu) {
+ 			mask = leaf_node_cpu_bit(rnp, cpu);
+ 			if (!(READ_ONCE(rnp->expmask) & mask))
+ 				continue;
+ 			preempt_disable(); // For smp_processor_id() in dump_cpu_task().
+ 			dump_cpu_task(cpu);
+ 			preempt_enable();
++			nbcon_cpu_emergency_flush();
+ 		}
+ 		rcu_exp_print_detail_task_stall_rnp(rnp);
+ 	}
+ }
+ 
  /*
   * Wait for the expedited grace period to elapse, issuing any needed
   * RCU CPU stall warnings along the way.
@@@ -597,60 -654,8 +659,11 @@@ static void synchronize_rcu_expedited_w
  		j = jiffies;
  		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_EXP, (void *)(j - jiffies_start));
  		trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
- 		pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
- 		       rcu_state.name);
- 		ndetected = 0;
- 		rcu_for_each_leaf_node(rnp) {
- 			ndetected += rcu_print_task_exp_stall(rnp);
- 			for_each_leaf_node_possible_cpu(rnp, cpu) {
- 				struct rcu_data *rdp;
- 
- 				mask = leaf_node_cpu_bit(rnp, cpu);
- 				if (!(READ_ONCE(rnp->expmask) & mask))
- 					continue;
- 				ndetected++;
- 				rdp = per_cpu_ptr(&rcu_data, cpu);
- 				pr_cont(" %d-%c%c%c%c", cpu,
- 					"O."[!!cpu_online(cpu)],
- 					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
- 					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
- 					"D."[!!data_race(rdp->cpu_no_qs.b.exp)]);
- 			}
- 		}
- 		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
- 			j - jiffies_start, rcu_state.expedited_sequence,
- 			data_race(rnp_root->expmask),
- 			".T"[!!data_race(rnp_root->exp_tasks)]);
- 		if (ndetected) {
- 			pr_err("blocking rcu_node structures (internal RCU debug):");
- 			rcu_for_each_node_breadth_first(rnp) {
- 				if (rnp == rnp_root)
- 					continue; /* printed unconditionally */
- 				if (sync_rcu_exp_done_unlocked(rnp))
- 					continue;
- 				pr_cont(" l=%u:%d-%d:%#lx/%c",
- 					rnp->level, rnp->grplo, rnp->grphi,
- 					data_race(rnp->expmask),
- 					".T"[!!data_race(rnp->exp_tasks)]);
- 			}
- 			pr_cont("\n");
- 		}
- 		rcu_for_each_leaf_node(rnp) {
- 			for_each_leaf_node_possible_cpu(rnp, cpu) {
- 				mask = leaf_node_cpu_bit(rnp, cpu);
- 				if (!(READ_ONCE(rnp->expmask) & mask))
- 					continue;
- 				preempt_disable(); // For smp_processor_id() in dump_cpu_task().
- 				dump_cpu_task(cpu);
- 				preempt_enable();
- 				nbcon_cpu_emergency_flush();
- 			}
- 			rcu_exp_print_detail_task_stall_rnp(rnp);
- 		}
+ 		synchronize_rcu_expedited_stall(jiffies_start, j);
  		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
 +
 +		nbcon_cpu_emergency_exit();
 +
  		panic_on_rcu_stall();
  	}
  }

On Wed, Jul 03, 2024 at 01:18:20PM +1000, Stephen Rothwell wrote:
> Hi all,
> 
> Today's linux-next merge of the paulmck tree got a conflict in:
> 
>   kernel/rcu/tree_exp.h
> 
> between commit:
> 
>   9a30ceb4d93e ("rcu: Mark emergency sections in rcu stalls")
> 
> from the printk tree and commit:
> 
>   c19a6725b019 ("rcu: Extract synchronize_rcu_expedited_stall() from synchronize_rcu_expedited_wait()")
> 
> from the paulmck tree.
> 
> I fixed it up (see below) and can carry the fix as necessary. This
> is now fixed as far as linux-next is concerned, but any non trivial
> conflicts should be mentioned to your upstream maintainer when your tree
> is submitted for merging.  You may also want to consider cooperating
> with the maintainer of the conflicting tree to minimise any particularly
> complex conflicts.

Thank you for catching this!

Given how near we are to the merge window, I will defer c19a6725b019 to
the v6.12 merge window.  Abundance of caution and all that...

							Thanx, Paul

> -- 
> Cheers,
> Stephen Rothwell
> 
> diff --cc kernel/rcu/tree_exp.h
> index be2d251e84f8,d4be644afb50..000000000000
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@@ -543,6 -542,69 +543,70 @@@ static bool synchronize_rcu_expedited_w
>   	return false;
>   }
>   
> + /*
> +  * Print out an expedited RCU CPU stall warning message.
> +  */
> + static void synchronize_rcu_expedited_stall(unsigned long jiffies_start, unsigned long j)
> + {
> + 	int cpu;
> + 	unsigned long mask;
> + 	int ndetected;
> + 	struct rcu_node *rnp;
> + 	struct rcu_node *rnp_root = rcu_get_root();
> + 
> + 	if (READ_ONCE(csd_lock_suppress_rcu_stall) && csd_lock_is_stuck()) {
> + 		pr_err("INFO: %s detected expedited stalls, but suppressed full report due to a stuck CSD-lock.\n", rcu_state.name);
> + 		return;
> + 	}
> + 	pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {", rcu_state.name);
> + 	ndetected = 0;
> + 	rcu_for_each_leaf_node(rnp) {
> + 		ndetected += rcu_print_task_exp_stall(rnp);
> + 		for_each_leaf_node_possible_cpu(rnp, cpu) {
> + 			struct rcu_data *rdp;
> + 
> + 			mask = leaf_node_cpu_bit(rnp, cpu);
> + 			if (!(READ_ONCE(rnp->expmask) & mask))
> + 				continue;
> + 			ndetected++;
> + 			rdp = per_cpu_ptr(&rcu_data, cpu);
> + 			pr_cont(" %d-%c%c%c%c", cpu,
> + 				"O."[!!cpu_online(cpu)],
> + 				"o."[!!(rdp->grpmask & rnp->expmaskinit)],
> + 				"N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
> + 				"D."[!!data_race(rdp->cpu_no_qs.b.exp)]);
> + 		}
> + 	}
> + 	pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
> + 		j - jiffies_start, rcu_state.expedited_sequence, data_race(rnp_root->expmask),
> + 		".T"[!!data_race(rnp_root->exp_tasks)]);
> + 	if (ndetected) {
> + 		pr_err("blocking rcu_node structures (internal RCU debug):");
> + 		rcu_for_each_node_breadth_first(rnp) {
> + 			if (rnp == rnp_root)
> + 				continue; /* printed unconditionally */
> + 			if (sync_rcu_exp_done_unlocked(rnp))
> + 				continue;
> + 			pr_cont(" l=%u:%d-%d:%#lx/%c",
> + 				rnp->level, rnp->grplo, rnp->grphi, data_race(rnp->expmask),
> + 				".T"[!!data_race(rnp->exp_tasks)]);
> + 		}
> + 		pr_cont("\n");
> + 	}
> + 	rcu_for_each_leaf_node(rnp) {
> + 		for_each_leaf_node_possible_cpu(rnp, cpu) {
> + 			mask = leaf_node_cpu_bit(rnp, cpu);
> + 			if (!(READ_ONCE(rnp->expmask) & mask))
> + 				continue;
> + 			preempt_disable(); // For smp_processor_id() in dump_cpu_task().
> + 			dump_cpu_task(cpu);
> + 			preempt_enable();
> ++			nbcon_cpu_emergency_flush();
> + 		}
> + 		rcu_exp_print_detail_task_stall_rnp(rnp);
> + 	}
> + }
> + 
>   /*
>    * Wait for the expedited grace period to elapse, issuing any needed
>    * RCU CPU stall warnings along the way.
> @@@ -597,60 -654,8 +659,11 @@@ static void synchronize_rcu_expedited_w
>   		j = jiffies;
>   		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_EXP, (void *)(j - jiffies_start));
>   		trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
> - 		pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
> - 		       rcu_state.name);
> - 		ndetected = 0;
> - 		rcu_for_each_leaf_node(rnp) {
> - 			ndetected += rcu_print_task_exp_stall(rnp);
> - 			for_each_leaf_node_possible_cpu(rnp, cpu) {
> - 				struct rcu_data *rdp;
> - 
> - 				mask = leaf_node_cpu_bit(rnp, cpu);
> - 				if (!(READ_ONCE(rnp->expmask) & mask))
> - 					continue;
> - 				ndetected++;
> - 				rdp = per_cpu_ptr(&rcu_data, cpu);
> - 				pr_cont(" %d-%c%c%c%c", cpu,
> - 					"O."[!!cpu_online(cpu)],
> - 					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
> - 					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
> - 					"D."[!!data_race(rdp->cpu_no_qs.b.exp)]);
> - 			}
> - 		}
> - 		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
> - 			j - jiffies_start, rcu_state.expedited_sequence,
> - 			data_race(rnp_root->expmask),
> - 			".T"[!!data_race(rnp_root->exp_tasks)]);
> - 		if (ndetected) {
> - 			pr_err("blocking rcu_node structures (internal RCU debug):");
> - 			rcu_for_each_node_breadth_first(rnp) {
> - 				if (rnp == rnp_root)
> - 					continue; /* printed unconditionally */
> - 				if (sync_rcu_exp_done_unlocked(rnp))
> - 					continue;
> - 				pr_cont(" l=%u:%d-%d:%#lx/%c",
> - 					rnp->level, rnp->grplo, rnp->grphi,
> - 					data_race(rnp->expmask),
> - 					".T"[!!data_race(rnp->exp_tasks)]);
> - 			}
> - 			pr_cont("\n");
> - 		}
> - 		rcu_for_each_leaf_node(rnp) {
> - 			for_each_leaf_node_possible_cpu(rnp, cpu) {
> - 				mask = leaf_node_cpu_bit(rnp, cpu);
> - 				if (!(READ_ONCE(rnp->expmask) & mask))
> - 					continue;
> - 				preempt_disable(); // For smp_processor_id() in dump_cpu_task().
> - 				dump_cpu_task(cpu);
> - 				preempt_enable();
> - 				nbcon_cpu_emergency_flush();
> - 			}
> - 			rcu_exp_print_detail_task_stall_rnp(rnp);
> - 		}
> + 		synchronize_rcu_expedited_stall(jiffies_start, j);
>   		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
>  +
>  +		nbcon_cpu_emergency_exit();
>  +
>   		panic_on_rcu_stall();
>   	}
>   }