[PATCH 2/3] perf_events: fix validation of events using an extrareg (v2)

[Stephane Eranian]

The validate_group() function needs to validate events with
extra shared regs. Within an event group, only events with
the same value for the extra reg can co-exist. This was not
checked by validate_group() because it was missing the
shared_regs logic.
    
This patch changes the allocation of the fake cpuc used for
validation to also point to a fake shared_regs structure such
that group events be properly testing.
    
The is_fake field is necessary to avoid a lockdep issue having
to do with interrupt masking and era->lock.
    
Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
---

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 0f8d3ff..3918927 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -139,6 +139,7 @@ struct cpu_hw_events {
 	struct perf_event	*event_list[X86_PMC_IDX_MAX]; /* in enabled order */
 
 	unsigned int		group_flag;
+	bool			is_fake;	/* fake cpuc for validation purposes */
 
 	/*
 	 * Intel DebugStore bits
@@ -1682,6 +1683,42 @@ static int x86_pmu_commit_txn(struct pmu *pmu)
 	perf_pmu_enable(pmu);
 	return 0;
 }
+/*
+ * a fake_cpuc is used to validate event groups. Due to
+ * the extra reg logic, we need to also allocate a fake
+ * per_core and per_cpu structure. Otherwise, group events
+ * using extra reg may conflict without the kernel being
+ * able to catch this when the last event gets added to
+ * the group.
+ */
+static void free_fake_cpuc(struct cpu_hw_events *cpuc)
+{
+	kfree(cpuc->shared_regs);
+	kfree(cpuc);
+}
+
+static struct cpu_hw_events *allocate_fake_cpuc(void)
+{
+	struct cpu_hw_events *cpuc;
+	int cpu = smp_processor_id();
+
+	cpuc = kzalloc(sizeof(*cpuc), GFP_KERNEL);
+	if (!cpuc)
+		return ERR_PTR(-ENOMEM);
+
+	cpuc->is_fake = true;
+
+	/* only needed, if we have extra_regs */
+	if (x86_pmu.extra_regs) {
+		cpuc->shared_regs = allocate_shared_regs(cpu);
+		if (!cpuc->shared_regs)
+			goto error;
+	}
+	return cpuc;
+error:
+	free_fake_cpuc(cpuc);
+	return ERR_PTR(-ENOMEM);
+}
 
 /*
  * validate that we can schedule this event
@@ -1692,9 +1729,9 @@ static int validate_event(struct perf_event *event)
 	struct event_constraint *c;
 	int ret = 0;
 
-	fake_cpuc = kmalloc(sizeof(*fake_cpuc), GFP_KERNEL | __GFP_ZERO);
-	if (!fake_cpuc)
-		return -ENOMEM;
+	fake_cpuc = allocate_fake_cpuc();
+	if (IS_ERR(fake_cpuc))
+		return PTR_ERR(fake_cpuc);
 
 	c = x86_pmu.get_event_constraints(fake_cpuc, event);
 
@@ -1704,7 +1741,7 @@ static int validate_event(struct perf_event *event)
 	if (x86_pmu.put_event_constraints)
 		x86_pmu.put_event_constraints(fake_cpuc, event);
 
-	kfree(fake_cpuc);
+	free_fake_cpuc(fake_cpuc);
 
 	return ret;
 }
@@ -1724,35 +1761,32 @@ static int validate_group(struct perf_event *event)
 {
 	struct perf_event *leader = event->group_leader;
 	struct cpu_hw_events *fake_cpuc;
-	int ret, n;
+	int ret = -ENOSPC, n;
 
-	ret = -ENOMEM;
-	fake_cpuc = kmalloc(sizeof(*fake_cpuc), GFP_KERNEL | __GFP_ZERO);
-	if (!fake_cpuc)
-		goto out;
+	fake_cpuc = allocate_fake_cpuc();
+	if (IS_ERR(fake_cpuc))
+		return PTR_ERR(fake_cpuc);
 	/*
 	 * the event is not yet connected with its
 	 * siblings therefore we must first collect
 	 * existing siblings, then add the new event
 	 * before we can simulate the scheduling
 	 */
-	ret = -ENOSPC;
 	n = collect_events(fake_cpuc, leader, true);
 	if (n < 0)
-		goto out_free;
+		goto out;
 
 	fake_cpuc->n_events = n;
 	n = collect_events(fake_cpuc, event, false);
 	if (n < 0)
-		goto out_free;
+		goto out;
 
 	fake_cpuc->n_events = n;
 
 	ret = x86_pmu.schedule_events(fake_cpuc, n, NULL);
 
-out_free:
-	kfree(fake_cpuc);
 out:
+	free_fake_cpuc(fake_cpuc);
 	return ret;
 }
 
diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
index 2d40f33..974eaf2 100644
--- a/arch/x86/kernel/cpu/perf_event_intel.c
+++ b/arch/x86/kernel/cpu/perf_event_intel.c
@@ -1029,12 +1029,21 @@ __intel_shared_reg_get_constraints(struct cpu_hw_events *cpuc,
 	struct er_account *era;
 
 	/* already allocated shared msr */
-	if (reg->alloc || !cpuc->shared_regs)
+	if (reg->alloc)
 		return &unconstrained;
 
 	era = &cpuc->shared_regs->regs[reg->idx];
-
-	raw_spin_lock(&era->lock);
+	/*
+	 * we do not lock in case we come here via validate_group(), i.e.,
+	 * fake cpuc, otherwise we trigger a lockdep issue. Locking is not
+	 * necessary with the fakecpu, as we are simply trying to validate
+	 * co-scheduling within a group.
+	 *
+	 * Note that locking is done even when the register is not shared
+	 * between CPUs but there will never be contention.
+	 */
+	if (!cpuc->is_fake)
+		raw_spin_lock(&era->lock);
 
 	if (!atomic_read(&era->ref) || era->config == reg->config) {
 
@@ -1058,7 +1067,8 @@ __intel_shared_reg_get_constraints(struct cpu_hw_events *cpuc,
 		 */
 		c = &unconstrained;
 	}
-	raw_spin_unlock(&era->lock);
+	if (!cpuc->is_fake)
+		raw_spin_unlock(&era->lock);
 
 	return c;
 }
--
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