Re: [perf] lockdep warning between cpu_add_remove_lock and &dev->mutex.

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jan 23, 2023 at 07:39:24PM +0900, Tetsuo Handa wrote:
> Hello.
> 
> I tried to apply below patch, and hit lockdep warning during boot.
> Can you break this dependency?

  cpu_add_remove_lock
    cpu_hotplug_lock
      pmus_lock
        dev->mutex		(pmu_dev_alloc)

vs

  dev->mutex
    cpu_add_remove_lock		(pci_device_probe)


Possibly something like this might do -- I'm not entirely sure it's
fully correct, needs a bit of auditing.

---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index eacc3702654d..d6b2265a9982 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -13570,9 +13570,9 @@ static void perf_event_exit_cpu_context(int cpu)
 {
 	struct perf_cpu_context *cpuctx;
 	struct perf_event_context *ctx;
+	int idx = srcu_read_lock(&pmus_srcu);
 
 	// XXX simplify cpuctx->online
-	mutex_lock(&pmus_lock);
 	cpuctx = per_cpu_ptr(&perf_cpu_context, cpu);
 	ctx = &cpuctx->ctx;
 
@@ -13581,7 +13581,7 @@ static void perf_event_exit_cpu_context(int cpu)
 	cpuctx->online = 0;
 	mutex_unlock(&ctx->mutex);
 	cpumask_clear_cpu(cpu, perf_online_mask);
-	mutex_unlock(&pmus_lock);
+	srcu_read_unlock(&pmus_srcu, idx);
 }
 #else
 
@@ -13593,10 +13594,11 @@ int perf_event_init_cpu(unsigned int cpu)
 {
 	struct perf_cpu_context *cpuctx;
 	struct perf_event_context *ctx;
+	int idx;
 
 	perf_swevent_init_cpu(cpu);
 
-	mutex_lock(&pmus_lock);
+	idx = srcu_read_lock(&pmus_srcu);
 	cpumask_set_cpu(cpu, perf_online_mask);
 	cpuctx = per_cpu_ptr(&perf_cpu_context, cpu);
 	ctx = &cpuctx->ctx;
@@ -13604,7 +13606,7 @@ int perf_event_init_cpu(unsigned int cpu)
 	mutex_lock(&ctx->mutex);
 	cpuctx->online = 1;
 	mutex_unlock(&ctx->mutex);
-	mutex_unlock(&pmus_lock);
+	srcu_read_unlock(&pmus_srcu, idx);
 
 	return 0;
 }