Re: [PATCH v2 tip/perf/core 1/2] uprobes: allow put_uprobe() from non-sleepable softirq context

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Mon, Oct 07, 2024 at 05:25:55PM -0700, Andrii Nakryiko wrote:
> Currently put_uprobe() might trigger mutex_lock()/mutex_unlock(), which
> makes it unsuitable to be called from more restricted context like softirq.

This is delayed_uprobe_lock, right?

So can't we do something like so instead? 

---
 kernel/events/uprobes.c | 40 +++++++++++++++++++++++-----------------
 1 file changed, 23 insertions(+), 17 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 2a0059464383..d17a9046de35 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -83,9 +83,11 @@ struct delayed_uprobe {
 	struct list_head list;
 	struct uprobe *uprobe;
 	struct mm_struct *mm;
+	struct rcu_head rcu;
 };
 
-static DEFINE_MUTEX(delayed_uprobe_lock);
+/* XXX global state; use per mm list instead ? */
+static DEFINE_SPINLOCK(delayed_uprobe_lock);
 static LIST_HEAD(delayed_uprobe_list);
 
 /*
@@ -289,9 +291,11 @@ delayed_uprobe_check(struct uprobe *uprobe, struct mm_struct *mm)
 {
 	struct delayed_uprobe *du;
 
-	list_for_each_entry(du, &delayed_uprobe_list, list)
+	guard(rcu)();
+	list_for_each_entry_rcu(du, &delayed_uprobe_list, list) {
 		if (du->uprobe == uprobe && du->mm == mm)
 			return du;
+	}
 	return NULL;
 }
 
@@ -308,7 +312,8 @@ static int delayed_uprobe_add(struct uprobe *uprobe, struct mm_struct *mm)
 
 	du->uprobe = uprobe;
 	du->mm = mm;
-	list_add(&du->list, &delayed_uprobe_list);
+	scoped_guard(spinlock, &delayed_uprobe_lock)
+		list_add_rcu(&du->list, &delayed_uprobe_list);
 	return 0;
 }
 
@@ -316,19 +321,21 @@ static void delayed_uprobe_delete(struct delayed_uprobe *du)
 {
 	if (WARN_ON(!du))
 		return;
-	list_del(&du->list);
-	kfree(du);
+	scoped_guard(spinlock, &delayed_uprobe_lock)
+		list_del(&du->list);
+	kfree_rcu(du, rcu);
 }
 
 static void delayed_uprobe_remove(struct uprobe *uprobe, struct mm_struct *mm)
 {
-	struct list_head *pos, *q;
 	struct delayed_uprobe *du;
+	struct list_head *pos;
 
 	if (!uprobe && !mm)
 		return;
 
-	list_for_each_safe(pos, q, &delayed_uprobe_list) {
+	guard(rcu)();
+	list_for_each_rcu(pos, &delayed_uprobe_list) {
 		du = list_entry(pos, struct delayed_uprobe, list);
 
 		if (uprobe && du->uprobe != uprobe)
@@ -434,12 +441,10 @@ static int update_ref_ctr(struct uprobe *uprobe, struct mm_struct *mm,
 			return ret;
 	}
 
-	mutex_lock(&delayed_uprobe_lock);
 	if (d > 0)
 		ret = delayed_uprobe_add(uprobe, mm);
 	else
 		delayed_uprobe_remove(uprobe, mm);
-	mutex_unlock(&delayed_uprobe_lock);
 
 	return ret;
 }
@@ -645,9 +650,7 @@ static void put_uprobe(struct uprobe *uprobe)
 	 * gets called, we don't get a chance to remove uprobe from
 	 * delayed_uprobe_list from remove_breakpoint(). Do it here.
 	 */
-	mutex_lock(&delayed_uprobe_lock);
 	delayed_uprobe_remove(uprobe, NULL);
-	mutex_unlock(&delayed_uprobe_lock);
 
 	call_rcu_tasks_trace(&uprobe->rcu, uprobe_free_rcu);
 }
@@ -1350,13 +1353,18 @@ static void build_probe_list(struct inode *inode,
 /* @vma contains reference counter, not the probed instruction. */
 static int delayed_ref_ctr_inc(struct vm_area_struct *vma)
 {
-	struct list_head *pos, *q;
 	struct delayed_uprobe *du;
+	struct list_head *pos;
 	unsigned long vaddr;
 	int ret = 0, err = 0;
 
-	mutex_lock(&delayed_uprobe_lock);
-	list_for_each_safe(pos, q, &delayed_uprobe_list) {
+	/*
+	 * delayed_uprobe_list is added to when the ref_ctr is not mapped
+	 * and is consulted (this function) when adding maps. And since
+	 * mmap_lock serializes these, it is not possible miss an entry.
+	 */
+	guard(rcu)();
+	list_for_each_rcu(pos, &delayed_uprobe_list) {
 		du = list_entry(pos, struct delayed_uprobe, list);
 
 		if (du->mm != vma->vm_mm ||
@@ -1370,9 +1378,9 @@ static int delayed_ref_ctr_inc(struct vm_area_struct *vma)
 			if (!err)
 				err = ret;
 		}
+
 		delayed_uprobe_delete(du);
 	}
-	mutex_unlock(&delayed_uprobe_lock);
 	return err;
 }
 
@@ -1596,9 +1604,7 @@ void uprobe_clear_state(struct mm_struct *mm)
 {
 	struct xol_area *area = mm->uprobes_state.xol_area;
 
-	mutex_lock(&delayed_uprobe_lock);
 	delayed_uprobe_remove(NULL, mm);
-	mutex_unlock(&delayed_uprobe_lock);
 
 	if (!area)
 		return;