Re: WARNING: possible circular locking dependency detected

[Peter Zijlstra]

On Tue, Aug 29, 2017 at 10:10:37PM +0200, Thomas Gleixner wrote:
> On Tue, 29 Aug 2017, Peter Zijlstra wrote:
> > So I have a patch _somewhere_ that preserves the event<->cpu relation
> > across hotplug and disable/enable would be sufficient. If you want I can
> > try and dig that out and make it work again.
> > 
> > That would avoid having to do the destroy/create cycle of the watchdog
> > events.
> 
> Yes, that would solve the x86_release_hw() issue, but still lots of the
> other rework is required in one way or the other.
> 
> I'm currently trying to avoid that extra lock mess in the cpu hotplug code,
> which would just open the door for everybody to add his extra locks there,
> so we end up taking a gazillion locks before we can hotplug :)
> 
> I think I have an idea how to solve that cleanly, but certainly your offer
> of preserving the event - cpu relation accross hotplug would help
> tremendously.

I think something like the below ought to work. Compile tested only.

On offline it basically does perf_event_disable() for all CPU context
events, and then adds HOTPLUG_OFFSET (-32) to arrive at: OFF +
HOTPLUG_OFFSET = -33.

That's smaller than ERROR and thus perf_event_enable() no-ops on events
for offline CPUs (maybe we should try and plumb an error return for
IOC_ENABLE).

On online we subtract the HOTPLUG_OFFSET again and the event becomes a
regular OFF, after which perf_event_enable() should work again.

---
 include/linux/perf_event.h |  2 ++
 kernel/events/core.c       | 51 +++++++++++++++++++++++++++++++++-------------
 2 files changed, 39 insertions(+), 14 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 9bac4bfa5e1a..7b39ceeb206b 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -497,6 +497,8 @@ enum perf_event_active_state {
 	PERF_EVENT_STATE_OFF		= -1,
 	PERF_EVENT_STATE_INACTIVE	=  0,
 	PERF_EVENT_STATE_ACTIVE		=  1,
+
+	PERF_EVENT_STATE_HOTPLUG_OFFSET	= -32,
 };
 
 struct file;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index f77c97477e08..b277c27fd81e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11025,19 +11025,21 @@ void perf_swevent_init_cpu(unsigned int cpu)
 }
 
 #if defined CONFIG_HOTPLUG_CPU || defined CONFIG_KEXEC_CORE
-static void __perf_event_exit_context(void *__info)
+static void __perf_event_exit_cpu(void *__info)
 {
-	struct perf_event_context *ctx = __info;
-	struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
+	struct perf_cpu_context *cpuctx = __info;
+	struct perf_event_context *ctx = &cpuctx->ctx;
 	struct perf_event *event;
 
 	raw_spin_lock(&ctx->lock);
-	list_for_each_entry(event, &ctx->event_list, event_entry)
-		__perf_remove_from_context(event, cpuctx, ctx, (void *)DETACH_GROUP);
+	list_for_each_entry(event, &ctx->event_list, event_entry) {
+		__perf_event_disable(event, cpuctx, ctx, NULL);
+		event->state += PERF_EVENT_STATE_HOTPLUG_OFFSET;
+	}
 	raw_spin_unlock(&ctx->lock);
 }
 
-static void perf_event_exit_cpu_context(int cpu)
+int perf_event_exit_cpu(unsigned int cpu)
 {
 	struct perf_cpu_context *cpuctx;
 	struct perf_event_context *ctx;
@@ -11049,17 +11051,43 @@ static void perf_event_exit_cpu_context(int cpu)
 		ctx = &cpuctx->ctx;
 
 		mutex_lock(&ctx->mutex);
-		smp_call_function_single(cpu, __perf_event_exit_context, ctx, 1);
+		smp_call_function_single(cpu, __perf_event_exit_cpu, cpuctx, 1);
 		cpuctx->online = 0;
 		mutex_unlock(&ctx->mutex);
 	}
 	cpumask_clear_cpu(cpu, perf_online_mask);
 	mutex_unlock(&pmus_lock);
+
+	return 0;
+}
+
+static void __perf_event_init_cpu(void *__info)
+{
+	struct perf_cpu_context *cpuctx = __info;
+	struct perf_event_context *ctx = &cpuctx->ctx;
+	struct perf_event *event;
+
+	raw_spin_lock(&ctx->lock);
+	list_for_each_entry(event, &ctx->event_list, event_entry)
+		event->state -= PERF_EVENT_STATE_HOTPLUG_OFFSET;
+	raw_spin_unlock(&ctx->lock);
+}
+
+static void _perf_event_init_cpu(int cpu, struct perf_cpu_context *cpuctx)
+{
+	smp_call_function_single(cpu, __perf_event_init_cpu, cpuctx, 1);
 }
+
 #else
 
-static void perf_event_exit_cpu_context(int cpu) { }
+int perf_event_exit_cpu(unsigned int cpu)
+{
+	return 0;
+}
 
+static void _perf_event_init_cpu(int cpu, struct perf_cpu_context *cpuctx)
+{
+}
 #endif
 
 int perf_event_init_cpu(unsigned int cpu)
@@ -11078,6 +11106,7 @@ int perf_event_init_cpu(unsigned int cpu)
 
 		mutex_lock(&ctx->mutex);
 		cpuctx->online = 1;
+		_perf_event_init_cpu(cpu, cpuctx);
 		mutex_unlock(&ctx->mutex);
 	}
 	mutex_unlock(&pmus_lock);
@@ -11085,12 +11114,6 @@ int perf_event_init_cpu(unsigned int cpu)
 	return 0;
 }
 
-int perf_event_exit_cpu(unsigned int cpu)
-{
-	perf_event_exit_cpu_context(cpu);
-	return 0;
-}
-
 static int
 perf_reboot(struct notifier_block *notifier, unsigned long val, void *v)
 {