batman-adv: bypass empty buckets in batadv_purge_orig_ref()

Many syzbot reports are pointing to soft lockups in
batadv_purge_orig_ref() [1]

Root cause is unknown, but we can avoid spending too much
time there and perhaps get more interesting reports.

[1]

watchdog: BUG: soft lockup - CPU#0 stuck for 27s! [kworker/u4:6:621]
Modules linked in:
irq event stamp: 6182794
 hardirqs last  enabled at (6182793): [<ffff8000801dae10>] __local_bh_enable_ip+0x224/0x44c kernel/softirq.c:386
 hardirqs last disabled at (6182794): [<ffff80008ad66a78>] __el1_irq arch/arm64/kernel/entry-common.c:533 [inline]
 hardirqs last disabled at (6182794): [<ffff80008ad66a78>] el1_interrupt+0x24/0x68 arch/arm64/kernel/entry-common.c:551
 softirqs last  enabled at (6182792): [<ffff80008aab71c4>] spin_unlock_bh include/linux/spinlock.h:396 [inline]
 softirqs last  enabled at (6182792): [<ffff80008aab71c4>] batadv_purge_orig_ref+0x114c/0x1228 net/batman-adv/originator.c:1287
 softirqs last disabled at (6182790): [<ffff80008aab61dc>] spin_lock_bh include/linux/spinlock.h:356 [inline]
 softirqs last disabled at (6182790): [<ffff80008aab61dc>] batadv_purge_orig_ref+0x164/0x1228 net/batman-adv/originator.c:1271
CPU: 0 PID: 621 Comm: kworker/u4:6 Not tainted 6.8.0-rc7-syzkaller-g707081b61156 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024
Workqueue: bat_events batadv_purge_orig
pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : should_resched arch/arm64/include/asm/preempt.h:79 [inline]
 pc : __local_bh_enable_ip+0x228/0x44c kernel/softirq.c:388
 lr : __local_bh_enable_ip+0x224/0x44c kernel/softirq.c:386
sp : ffff800099007970
x29: ffff800099007980 x28: 1fffe00018fce1bd x27: dfff800000000000
x26: ffff0000d2620008 x25: ffff0000c7e70de8 x24: 0000000000000001
x23: 1fffe00018e57781 x22: dfff800000000000 x21: ffff80008aab71c4
x20: ffff0001b40136c0 x19: ffff0000c72bbc08 x18: 1fffe0001a817bb0
x17: ffff800125414000 x16: ffff80008032116c x15: 0000000000000001
x14: 1fffe0001ee9d610 x13: 0000000000000000 x12: 0000000000000003
x11: 0000000000000000 x10: 0000000000ff0100 x9 : 0000000000000000
x8 : 00000000005e5789 x7 : ffff80008aab61dc x6 : 0000000000000000
x5 : 0000000000000000 x4 : 0000000000000001 x3 : 0000000000000000
x2 : 0000000000000006 x1 : 0000000000000080 x0 : ffff800125414000
Call trace:
  __daif_local_irq_enable arch/arm64/include/asm/irqflags.h:27 [inline]
  arch_local_irq_enable arch/arm64/include/asm/irqflags.h:49 [inline]
  __local_bh_enable_ip+0x228/0x44c kernel/softirq.c:386
  __raw_spin_unlock_bh include/linux/spinlock_api_smp.h:167 [inline]
  _raw_spin_unlock_bh+0x3c/0x4c kernel/locking/spinlock.c:210
  spin_unlock_bh include/linux/spinlock.h:396 [inline]
  batadv_purge_orig_ref+0x114c/0x1228 net/batman-adv/originator.c:1287
  batadv_purge_orig+0x20/0x70 net/batman-adv/originator.c:1300
  process_one_work+0x694/0x1204 kernel/workqueue.c:2633
  process_scheduled_works kernel/workqueue.c:2706 [inline]
  worker_thread+0x938/0xef4 kernel/workqueue.c:2787
  kthread+0x288/0x310 kernel/kthread.c:388
  ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:860
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 6.8.0-rc7-syzkaller-g707081b61156 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024
pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : arch_local_irq_enable+0x8/0xc arch/arm64/include/asm/irqflags.h:51
 lr : default_idle_call+0xf8/0x128 kernel/sched/idle.c:103
sp : ffff800093a17d30
x29: ffff800093a17d30 x28: dfff800000000000 x27: 1ffff00012742fb4
x26: ffff80008ec9d000 x25: 0000000000000000 x24: 0000000000000002
x23: 1ffff00011d93a74 x22: ffff80008ec9d3a0 x21: 0000000000000000
x20: ffff0000c19dbc00 x19: ffff8000802d0fd8 x18: 1fffe00036804396
x17: ffff80008ec9d000 x16: ffff8000802d089c x15: 0000000000000001
x14: 1fffe00036805f10 x13: 0000000000000000 x12: 0000000000000003
x11: 0000000000000001 x10: 0000000000000003 x9 : 0000000000000000
x8 : 00000000000ce8d1 x7 : ffff8000804609e4 x6 : 0000000000000000
x5 : 0000000000000001 x4 : 0000000000000001 x3 : ffff80008ad6aac0
x2 : 0000000000000000 x1 : ffff80008aedea60 x0 : ffff800125436000
Call trace:
  __daif_local_irq_enable arch/arm64/include/asm/irqflags.h:27 [inline]
  arch_local_irq_enable+0x8/0xc arch/arm64/include/asm/irqflags.h:49
  cpuidle_idle_call kernel/sched/idle.c:170 [inline]
  do_idle+0x1f0/0x4e8 kernel/sched/idle.c:312
  cpu_startup_entry+0x5c/0x74 kernel/sched/idle.c:410
  secondary_start_kernel+0x198/0x1c0 arch/arm64/kernel/smp.c:272
  __secondary_switched+0xb8/0xbc arch/arm64/kernel/head.S:404

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/originator.c

@@ -1266,6 +1266,8 @@ void batadv_purge_orig_ref(struct batadv_priv *bat_priv)
 	/* for all origins... */
 	for (i = 0; i < hash->size; i++) {
 		head = &hash->table[i];
+		if (hlist_empty(head))
+			continue;
 		list_lock = &hash->list_locks[i];
 
 		spin_lock_bh(list_lock);