bpf: Fix infinite loop in pcpu_freelist push with one possible CPU

__pcpu_freelist_push() can loop forever when only one CPU is possible
and an NMI re-enters pcpu_freelist_push() while the interrupted context
holds that CPU's freelist lock.

After the current-CPU fast path fails, the fallback loop walks
cpu_possible_mask while skipping the current CPU. With CONFIG_SMP=n, or
when an SMP kernel is limited to one possible CPU with nr_cpus=1 or
possible_cpus=1, there are no other possible CPUs to examine. The loop
therefore makes no lock acquisition attempt and can never make progress.

The following stack was observed on a UP system:

  NMI context:
    pcpu_freelist_push
    free_htab_elem
    htab_map_delete_elem
    [perf-event BPF program]
    __perf_event_overflow
    perf_event_nmi_handler
    exc_nmi

  Interrupted context:
    __pcpu_freelist_push
    pcpu_freelist_push
    free_htab_elem
    htab_map_delete_elem
    [raw_tp/sys_enter BPF program]
    __bpf_trace_sys_enter
    do_syscall_64

raw_res_spin_lock() detects the same-CPU recursive acquisition and
returns -EDEADLK, but the subsequent fallback loop has no candidate head
on a system with one possible CPU.

Restore the extra fallback head that existed before the rqspinlock
conversion. Keep the current-CPU fast path, then try the other possible
CPUs and finally the extra head. The additional head lets a push, which
cannot fail without losing a preallocated element, make progress when the
only per-CPU head is held by the interrupted context.

Also check the extra head from the pop path so that nodes placed there
can be reused.

Fixes: f2ac0e5d1c ("bpf: Convert percpu_freelist.c to rqspinlock")
Signed-off-by: Hui Su <sh_def@163.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/20260806175600.1993595-1-sh_def@163.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

kernel/bpf/percpu_freelist.c

@@ -17,6 +17,8 @@ int pcpu_freelist_init(struct pcpu_freelist *s)
 		raw_res_spin_lock_init(&head->lock);
 		head->first = NULL;
 	}
+	raw_res_spin_lock_init(&s->extralist.lock);
+	s->extralist.first = NULL;
 	return 0;
 }
 
@@ -46,22 +48,28 @@ void __pcpu_freelist_push(struct pcpu_freelist *s,
 			struct pcpu_freelist_node *node)
 {
 	struct pcpu_freelist_head *head;
-	int cpu;
+	int cpu, this_cpu;
 
 	if (___pcpu_freelist_push(this_cpu_ptr(s->freelist), node))
 		return;
 
+	this_cpu = raw_smp_processor_id();
 	while (true) {
-		for_each_cpu_wrap(cpu, cpu_possible_mask, raw_smp_processor_id()) {
-			if (cpu == raw_smp_processor_id())
+		for_each_cpu_wrap(cpu, cpu_possible_mask, this_cpu) {
+			if (cpu == this_cpu)
 				continue;
+
 			head = per_cpu_ptr(s->freelist, cpu);
-			if (raw_res_spin_lock(&head->lock))
-				continue;
-			pcpu_freelist_push_node(head, node);
-			raw_res_spin_unlock(&head->lock);
-			return;
+			if (___pcpu_freelist_push(head, node))
+				return;
 		}
+
+		/*
+		 * Push cannot fail. Use the extra list when none of the
+		 * per-CPU freelists can accept the node.
+		 */
+		if (___pcpu_freelist_push(&s->extralist, node))
+			return;
 	}
 }
 
@@ -117,6 +125,17 @@ static struct pcpu_freelist_node *___pcpu_freelist_pop(struct pcpu_freelist *s)
 		}
 		raw_res_spin_unlock(&head->lock);
 	}
+
+	/* Per-CPU lists are empty or unavailable, try the extra list. */
+	head = &s->extralist;
+	if (!READ_ONCE(head->first))
+		return NULL;
+	if (raw_res_spin_lock(&head->lock))
+		return NULL;
+	node = head->first;
+	if (node)
+		WRITE_ONCE(head->first, node->next);
+	raw_res_spin_unlock(&head->lock);
 	return node;
 }
 

kernel/bpf/percpu_freelist.h

@@ -14,6 +14,7 @@ struct pcpu_freelist_head {
 
 struct pcpu_freelist {
 	struct pcpu_freelist_head __percpu *freelist;
+	struct pcpu_freelist_head extralist;
 };
 
 struct pcpu_freelist_node {