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Date:   Mon, 15 May 2017 11:03:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 0/5] perf/tracing/cpuhotplug: Fix locking order

On Sat, May 13, 2017 at 06:40:03AM -0700, Paul E. McKenney wrote:
> On Fri, May 12, 2017 at 05:34:48PM -0400, Steven Rostedt wrote:
> > On Fri, 12 May 2017 21:49:56 +0200
> 
> [ . . . ]
> 
> > This means that text_mutex, which was taken by the alternative code, no
> > longer is taken in cpu hotplug code. That means there's no longer a
> > deadlock scenario, as we don't have anyplace(*) that grabs
> > get_online_cpus() and takes the text_mutex. Removing that will
> > simplify things tremendously!
> > 
> > (*) with one exception: perf.
> > 
> > Currently perf does a get_online_cpu() at a high level. Will it be
> > possible to move that down, such that we don't have it taken when we do
> > any software events?
> 
> Can perf get rid of get_online_cpus(), perhaps using the mutexes acquired
> by perf_event_init_cpu() or by perf_event_exit_cpu()?


Best I could come up with is something like that below that moves the
get_online_cpus() to a possibly less onerous place.

Compile tested only..

---
 include/linux/perf_event.h |  2 ++
 kernel/events/core.c       | 85 ++++++++++++++++++++++++++++++++++------------
 2 files changed, 66 insertions(+), 21 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 24a635887f28..7d6aa29094b2 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -801,6 +801,8 @@ struct perf_cpu_context {
 
 	struct list_head		sched_cb_entry;
 	int				sched_cb_usage;
+
+	int				online;
 };
 
 struct perf_output_handle {
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 13f5b942580b..f9a595641d32 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3812,14 +3812,6 @@ find_get_context(struct pmu *pmu, struct task_struct *task,
 		if (perf_paranoid_cpu() && !capable(CAP_SYS_ADMIN))
 			return ERR_PTR(-EACCES);
 
-		/*
-		 * We could be clever and allow to attach a event to an
-		 * offline CPU and activate it when the CPU comes up, but
-		 * that's for later.
-		 */
-		if (!cpu_online(cpu))
-			return ERR_PTR(-ENODEV);
-
 		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
 		ctx = &cpuctx->ctx;
 		get_ctx(ctx);
@@ -9005,6 +8997,7 @@ int perf_pmu_register(struct pmu *pmu, const char *name, int type)
 {
 	int cpu, ret;
 
+	get_online_cpus();
 	mutex_lock(&pmus_lock);
 	ret = -ENOMEM;
 	pmu->pmu_disable_count = alloc_percpu(int);
@@ -9064,6 +9057,7 @@ int perf_pmu_register(struct pmu *pmu, const char *name, int type)
 		lockdep_set_class(&cpuctx->ctx.mutex, &cpuctx_mutex);
 		lockdep_set_class(&cpuctx->ctx.lock, &cpuctx_lock);
 		cpuctx->ctx.pmu = pmu;
+		cpuctx->online = cpu_online(cpu);
 
 		__perf_mux_hrtimer_init(cpuctx, cpu);
 	}
@@ -9099,6 +9093,7 @@ int perf_pmu_register(struct pmu *pmu, const char *name, int type)
 	ret = 0;
 unlock:
 	mutex_unlock(&pmus_lock);
+	put_online_cpus();
 
 	return ret;
 
@@ -9908,13 +9903,11 @@ SYSCALL_DEFINE5(perf_event_open,
 	if (flags & PERF_FLAG_PID_CGROUP)
 		cgroup_fd = pid;
 
-	get_online_cpus();
-
 	event = perf_event_alloc(&attr, cpu, task, group_leader, NULL,
 				 NULL, NULL, cgroup_fd);
 	if (IS_ERR(event)) {
 		err = PTR_ERR(event);
-		goto err_cpus;
+		goto err_cred;
 	}
 
 	if (is_sampling_event(event)) {
@@ -10078,6 +10071,23 @@ SYSCALL_DEFINE5(perf_event_open,
 		goto err_locked;
 	}
 
+	if (!task) {
+		/*
+		 * Check if the @cpu we're creating an event for is online.
+		 *
+		 * We use the perf_cpu_context::ctx::mutex to serialize against
+		 * the hotplug notifiers. See perf_event_{init,exit}_cpu().
+		 */
+		struct perf_cpu_context *cpuctx =
+			container_of(ctx, struct perf_cpu_context, ctx);
+
+		if (!cpuctx->online) {
+			err = -ENODEV;
+			goto err_locked;
+		}
+	}
+
+
 	/*
 	 * Must be under the same ctx::mutex as perf_install_in_context(),
 	 * because we need to serialize with concurrent event creation.
@@ -10162,8 +10172,6 @@ SYSCALL_DEFINE5(perf_event_open,
 		perf_event_ctx_unlock(group_leader, gctx);
 	mutex_unlock(&ctx->mutex);
 
-	put_online_cpus();
-
 	if (task) {
 		mutex_unlock(&task->signal->cred_guard_mutex);
 		put_task_struct(task);
@@ -10205,8 +10213,6 @@ SYSCALL_DEFINE5(perf_event_open,
 	 */
 	if (!event_file)
 		free_event(event);
-err_cpus:
-	put_online_cpus();
 err_cred:
 	if (task)
 		mutex_unlock(&task->signal->cred_guard_mutex);
@@ -10237,7 +10243,6 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
 	struct perf_event *event;
 	int err;
 
-	get_online_cpus();
 	/*
 	 * Get the target context (task or percpu):
 	 */
@@ -10264,6 +10269,21 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
 		goto err_unlock;
 	}
 
+	if (!task) {
+		/*
+		 * Check if the @cpu we're creating an event for is online.
+		 *
+		 * We use the perf_cpu_context::ctx::mutex to serialize against
+		 * the hotplug notifiers. See perf_event_{init,exit}_cpu().
+		 */
+		struct perf_cpu_context *cpuctx =
+			container_of(ctx, struct perf_cpu_context, ctx);
+		if (!cpuctx->online) {
+			err = -ENODEV;
+			goto err_unlock;
+		}
+	}
+
 	if (!exclusive_event_installable(event, ctx)) {
 		err = -EBUSY;
 		goto err_unlock;
@@ -10272,7 +10292,6 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
 	perf_install_in_context(ctx, event, cpu);
 	perf_unpin_context(ctx);
 	mutex_unlock(&ctx->mutex);
-	put_online_cpus();
 	return event;
 
 err_unlock:
@@ -10282,7 +10301,6 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
 err_free:
 	free_event(event);
 err:
-	put_online_cpus();
 	return ERR_PTR(err);
 }
 EXPORT_SYMBOL_GPL(perf_event_create_kernel_counter);
@@ -10951,7 +10969,7 @@ static void __init perf_event_init_all_cpus(void)
 	}
 }
 
-int perf_event_init_cpu(unsigned int cpu)
+void perf_swevent_init_cpu(unsigned int cpu)
 {
 	struct swevent_htable *swhash = &per_cpu(swevent_htable, cpu);
 
@@ -10964,7 +10982,6 @@ int perf_event_init_cpu(unsigned int cpu)
 		rcu_assign_pointer(swhash->swevent_hlist, hlist);
 	}
 	mutex_unlock(&swhash->hlist_mutex);
-	return 0;
 }
 
 #if defined CONFIG_HOTPLUG_CPU || defined CONFIG_KEXEC_CORE
@@ -10982,16 +10999,19 @@ static void __perf_event_exit_context(void *__info)
 
 static void perf_event_exit_cpu_context(int cpu)
 {
+	struct perf_cpu_context *cpuctx;
 	struct perf_event_context *ctx;
 	struct pmu *pmu;
 	int idx;
 
 	idx = srcu_read_lock(&pmus_srcu);
 	list_for_each_entry_rcu(pmu, &pmus, entry) {
-		ctx = &per_cpu_ptr(pmu->pmu_cpu_context, cpu)->ctx;
+		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
+		ctx = &cpuctx->ctx;
 
 		mutex_lock(&ctx->mutex);
 		smp_call_function_single(cpu, __perf_event_exit_context, ctx, 1);
+		cpuctx->online = 0;
 		mutex_unlock(&ctx->mutex);
 	}
 	srcu_read_unlock(&pmus_srcu, idx);
@@ -11002,6 +11022,29 @@ static void perf_event_exit_cpu_context(int cpu) { }
 
 #endif
 
+int perf_event_init_cpu(unsigned int cpu)
+{
+	struct perf_cpu_context *cpuctx;
+	struct perf_event_context *ctx;
+	struct pmu *pmu;
+	int idx;
+
+	perf_swevent_init_cpu(cpu);
+
+	idx = srcu_read_lock(&pmus_srcu);
+	list_for_each_entry_rcu(pmu, &pmus, entry) {
+		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
+		ctx = &cpuctx->ctx;
+
+		mutex_lock(&ctx->mutex);
+		cpuctx->online = 1;
+		mutex_unlock(&ctx->mutex);
+	}
+	srcu_read_unlock(&pmus_srcu, idx);
+
+	return 0;
+}
+
 int perf_event_exit_cpu(unsigned int cpu)
 {
 	perf_event_exit_cpu_context(cpu);

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