bpf: use raw_spin_trylock() for pcpu_freelist_push/pop in NMI

Recent improvements in LOCKDEP highlighted a potential A-A deadlock with pcpu_freelist in NMI: ./tools/testing/selftests/bpf/test_progs -t stacktrace_build_id_nmi [ 18.984807] ================================ [ 18.984807] WARNING: inconsistent lock state [ 18.984808] 5.9.0-rc6-01771-g1466de1330e1 #2967 Not tainted [ 18.984809] -------------------------------- [ 18.984809] inconsistent {INITIAL USE} -> {IN-NMI} usage. [ 18.984810] test_progs/1990 [HC2[2]:SC0[0]:HE0:SE1] takes: [ 18.984810] ffffe8ffffc219c0 (&head->lock){....}-{2:2}, at: __pcpu_freelist_pop+0xe3/0x180 [ 18.984813] {INITIAL USE} state was registered at: [ 18.984814] lock_acquire+0x175/0x7c0 [ 18.984814] _raw_spin_lock+0x2c/0x40 [ 18.984815] __pcpu_freelist_pop+0xe3/0x180 [ 18.984815] pcpu_freelist_pop+0x31/0x40 [ 18.984816] htab_map_alloc+0xbbf/0xf40 [ 18.984816] __do_sys_bpf+0x5aa/0x3ed0 [ 18.984817] do_syscall_64+0x2d/0x40 [ 18.984818] entry_SYSCALL_64_after_hwframe+0x44/0xa9 [ 18.984818] irq event stamp: 12 [ ... ] [ 18.984822] other info that might help us debug this: [ 18.984823] Possible unsafe locking scenario: [ 18.984823] [ 18.984824] CPU0 [ 18.984824] ---- [ 18.984824] lock(&head->lock); [ 18.984826] <Interrupt> [ 18.984826] lock(&head->lock); [ 18.984827] [ 18.984828] *** DEADLOCK *** [ 18.984828] [ 18.984829] 2 locks held by test_progs/1990: [ ... ] [ 18.984838] <NMI> [ 18.984838] dump_stack+0x9a/0xd0 [ 18.984839] lock_acquire+0x5c9/0x7c0 [ 18.984839] ? lock_release+0x6f0/0x6f0 [ 18.984840] ? __pcpu_freelist_pop+0xe3/0x180 [ 18.984840] _raw_spin_lock+0x2c/0x40 [ 18.984841] ? __pcpu_freelist_pop+0xe3/0x180 [ 18.984841] __pcpu_freelist_pop+0xe3/0x180 [ 18.984842] pcpu_freelist_pop+0x17/0x40 [ 18.984842] ? lock_release+0x6f0/0x6f0 [ 18.984843] __bpf_get_stackid+0x534/0xaf0 [ 18.984843] bpf_prog_1fd9e30e1438d3c5_oncpu+0x73/0x350 [ 18.984844] bpf_overflow_handler+0x12f/0x3f0 This is because pcpu_freelist_head.lock is accessed in both NMI and non-NMI context. Fix this issue by using raw_spin_trylock() in NMI. Since NMI interrupts non-NMI context, when NMI context tries to lock the raw_spinlock, non-NMI context of the same cpu may already have locked a lock and is blocked from unlocking the lock. For a system with N cpus, there could be N NMIs at the same time, and they may block N non-NMI raw_spinlocks. This is tricky for pcpu_freelist_push(), where unlike _pop(), failing _push() means leaking memory. This issue is more likely to trigger in non-SMP system. Fix this issue with an extra list, pcpu_freelist.extralist. The extralist is primarily used to take _push() when raw_spin_trylock() failed on all the per cpu lists. It should be empty most of the time. The following table summarizes the behavior of pcpu_freelist in NMI and non-NMI: non-NMI pop():	use _lock(); check per cpu lists first; if all per cpu lists are empty, check extralist; if extralist is empty, return NULL. non-NMI push(): use _lock(); only push to per cpu lists. NMI pop(): use _trylock(); check per cpu lists first; if all per cpu lists are locked or empty, check extralist; if extralist is locked or empty, return NULL. NMI push(): use _trylock(); check per cpu lists first; if all per cpu lists are locked; try push to extralist; if extralist is also locked, keep trying on per cpu lists. Reported-by: Alexei Starovoitov <ast@kernel.org> Signed-off-by: Song Liu <songliubraving@fb.com>

Author: liu-song-6

kernel/bpf/percpu_freelist.c

@@ -17,6 +17,8 @@ int pcpu_freelist_init(struct pcpu_freelist *s)
 raw_spin_lock_init(&head->lock);
 head->first = NULL;
 }
+raw_spin_lock_init(&s->extralist.lock);
+s->extralist.first = NULL;
 return 0;
 }
 
@@ -40,12 +42,50 @@ static inline void ___pcpu_freelist_push(struct pcpu_freelist_head *head,
 raw_spin_unlock(&head->lock);
 }
 
+static inline bool pcpu_freelist_try_push_extra(struct pcpu_freelist *s,
+struct pcpu_freelist_node *node)
+{
+if (!raw_spin_trylock(&s->extralist.lock))
+return false;
+
+pcpu_freelist_push_node(&s->extralist, node);
+raw_spin_unlock(&s->extralist.lock);
+return true;
+}
+
+static inline void ___pcpu_freelist_push_nmi(struct pcpu_freelist *s,
+ struct pcpu_freelist_node *node)
+{
+int cpu, orig_cpu;
+
+orig_cpu = cpu = raw_smp_processor_id();
+while (1) {
+struct pcpu_freelist_head *head;
+
+head = per_cpu_ptr(s->freelist, cpu);
+if (raw_spin_trylock(&head->lock)) {
+pcpu_freelist_push_node(head, node);
+raw_spin_unlock(&head->lock);
+return;
+}
+cpu = cpumask_next(cpu, cpu_possible_mask);
+if (cpu >= nr_cpu_ids)
+cpu = 0;
+
+/* cannot lock any per cpu lock, try extralist */
+if (cpu == orig_cpu &&
+ pcpu_freelist_try_push_extra(s, node))
+return;
+}
+}
+
 void __pcpu_freelist_push(struct pcpu_freelist *s,
 struct pcpu_freelist_node *node)
 {
-struct pcpu_freelist_head *head = this_cpu_ptr(s->freelist);
-
-___pcpu_freelist_push(head, node);
+if (in_nmi())
+___pcpu_freelist_push_nmi(s, node);
+else
+___pcpu_freelist_push(this_cpu_ptr(s->freelist), node);
 }
 
 void pcpu_freelist_push(struct pcpu_freelist *s,
@@ -81,7 +121,7 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
 }
 }
 
-struct pcpu_freelist_node *__pcpu_freelist_pop(struct pcpu_freelist *s)
+static struct pcpu_freelist_node *___pcpu_freelist_pop(struct pcpu_freelist *s)
 {
 struct pcpu_freelist_head *head;
 struct pcpu_freelist_node *node;
@@ -102,8 +142,59 @@ struct pcpu_freelist_node *__pcpu_freelist_pop(struct pcpu_freelist *s)
 if (cpu >= nr_cpu_ids)
 cpu = 0;
 if (cpu == orig_cpu)
-return NULL;
+break;
+}
+
+/* per cpu lists are all empty, try extralist */
+raw_spin_lock(&s->extralist.lock);
+node = s->extralist.first;
+if (node)
+s->extralist.first = node->next;
+raw_spin_unlock(&s->extralist.lock);
+return node;
+}
+
+static struct pcpu_freelist_node *
+___pcpu_freelist_pop_nmi(struct pcpu_freelist *s)
+{
+struct pcpu_freelist_head *head;
+struct pcpu_freelist_node *node;
+int orig_cpu, cpu;
+
+orig_cpu = cpu = raw_smp_processor_id();
+while (1) {
+head = per_cpu_ptr(s->freelist, cpu);
+if (raw_spin_trylock(&head->lock)) {
+node = head->first;
+if (node) {
+head->first = node->next;
+raw_spin_unlock(&head->lock);
+return node;
+}
+raw_spin_unlock(&head->lock);
+}
+cpu = cpumask_next(cpu, cpu_possible_mask);
+if (cpu >= nr_cpu_ids)
+cpu = 0;
+if (cpu == orig_cpu)
+break;
 }
+
+/* cannot pop from per cpu lists, try extralist */
+if (!raw_spin_trylock(&s->extralist.lock))
+return NULL;
+node = s->extralist.first;
+if (node)
+s->extralist.first = node->next;
+raw_spin_unlock(&s->extralist.lock);
+return node;
+}
+
+struct pcpu_freelist_node *__pcpu_freelist_pop(struct pcpu_freelist *s)
+{
+if (in_nmi())
+return ___pcpu_freelist_pop_nmi(s);
+return ___pcpu_freelist_pop(s);
 }
 
 struct pcpu_freelist_node *pcpu_freelist_pop(struct pcpu_freelist *s)

kernel/bpf/percpu_freelist.h

@@ -13,6 +13,7 @@ struct pcpu_freelist_head {
 
 struct pcpu_freelist {
 struct pcpu_freelist_head __percpu *freelist;
+struct pcpu_freelist_head extralist;
 };
 
 struct pcpu_freelist_node {