syzbot reported the following nfc_llcp_send_ui_frame() lockup:

The kernel is CONFIG_PREEMPT_VOLUNTARY=y, llcp_sock_sendmsg() stuck
in an infinite error reporting loop, because the system is low memory
and MSG_DONTWAIT nfc_alloc_send_skb() allocations fail:

        do {
        ...
                pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, MSG_DONTWAIT,
                                         frag_len + LLCP_HEADER_SIZE, &err);
                if (pdu == NULL) {
                        pr_err("Could not allocate PDU\n");
                        continue;
                }
        ...
        } while (remaining_len > 0);

nfc_llcp_send_ui_frame() spent enough time (94+ sec) trying to
allocate PDU, which resulted in RCU stall due to PREEMPT_VOLUNTARY:

 llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
 llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
...
 llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
 llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
 INFO: rcu_sched self-detected stall on CPU
         1-....: (20918 ticks this GP) idle=55a/1/4611686018427387906
 softirq=11347/11347 fqs=20240
          (t=125005 jiffies g=5572 c=5571 q=149)
 NMI backtrace for cpu 1
 CPU: 1 PID: 4811 Comm: syz-executor0 Not tainted 4.18.0-rc1+ #115
 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google
 01/01/2011
 Call Trace:
  <IRQ>
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
  nmi_cpu_backtrace.cold.4+0x19/0xce lib/nmi_backtrace.c:103
  nmi_trigger_cpumask_backtrace+0x151/0x192 lib/nmi_backtrace.c:62
  arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
  trigger_single_cpu_backtrace include/linux/nmi.h:156 [inline]
  rcu_dump_cpu_stacks+0x175/0x1c2 kernel/rcu/tree.c:1336
  print_cpu_stall kernel/rcu/tree.c:1485 [inline]
  check_cpu_stall.isra.60.cold.78+0x36c/0x5a6 kernel/rcu/tree.c:1553
  __rcu_pending kernel/rcu/tree.c:3244 [inline]
  rcu_pending kernel/rcu/tree.c:3291 [inline]
  rcu_check_callbacks+0x23f/0xcd0 kernel/rcu/tree.c:2646
  update_process_times+0x2d/0x70 kernel/time/timer.c:1636
  tick_sched_handle+0x9f/0x180 kernel/time/tick-sched.c:164
  tick_sched_timer+0x45/0x130 kernel/time/tick-sched.c:1274
  __run_hrtimer kernel/time/hrtimer.c:1398 [inline]
  __hrtimer_run_queues+0x3eb/0x10c0 kernel/time/hrtimer.c:1460
  hrtimer_interrupt+0x2f3/0x750 kernel/time/hrtimer.c:1518
  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1025 [inline]
  smp_apic_timer_interrupt+0x165/0x730 arch/x86/kernel/apic/apic.c:1050
  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:863
  </IRQ>
 RIP: 0010:arch_local_irq_restore arch/x86/include/asm/paravirt.h:783 [inline]
 RIP: 0010:console_unlock+0xc84/0x10b0 kernel/printk/printk.c:2397
 Code: c1 e8 03 42 80 3c 38 00 0f 85 bd 03 00 00 48 83 3d 38 f7 8e 07 00 0f 84
 69 02 00 00 e8 45 56 19 00 48 8b bd b0 fe ff ff 57 9d <0f> 1f 44 00 00 e9 96
 f5 ff ff e8 2d 56 19 00 48 8b 7d 08 e8 94 cf
 RSP: 0018:ffff8801aab0f358 EFLAGS: 00000293 ORIG_RAX: ffffffffffffff13
 RAX: ffff8801aa2802c0 RBX: 0000000000000200 RCX: 1ffff10035450163
 RDX: 0000000000000000 RSI: ffffffff8162b8fb RDI: 0000000000000293
 RBP: ffff8801aab0f4c0 R08: ffff8801aa280af8 R09: 0000000000000006
 R10: ffff8801aa2802c0 R11: 0000000000000000 R12: 0000000000000000
 R13: ffffffff84ea9880 R14: 0000000000000001 R15: dffffc0000000000
  vprintk_emit+0x6c6/0xdf0 kernel/printk/printk.c:1907
  vprintk_default+0x28/0x30 kernel/printk/printk.c:1948
  vprintk_func+0x7a/0xe7 kernel/printk/printk_safe.c:382
  printk+0xa7/0xcf kernel/printk/printk.c:1981
  nfc_llcp_send_ui_frame.cold.9+0x18/0x1f net/nfc/llcp_commands.c:758
  llcp_sock_sendmsg+0x278/0x350 net/nfc/llcp_sock.c:786
  sock_sendmsg_nosec net/socket.c:645 [inline]
  sock_sendmsg+0xd5/0x120 net/socket.c:655
  ___sys_sendmsg+0x51d/0x930 net/socket.c:2161
  __sys_sendmmsg+0x240/0x6f0 net/socket.c:2256
  __do_sys_sendmmsg net/socket.c:2285 [inline]
  __se_sys_sendmmsg net/socket.c:2282 [inline]
  __x64_sys_sendmmsg+0x9d/0x100 net/socket.c:2282
  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Address the issues by rate limiting nfc_alloc_send_skb() allocation
error, to avoid logbuf pollution, and do cond_resched() before llcp
attempts to allocate PDU again.

Signed-off-by: Sergey Senozhatsky <redacted>
Reported-by: syzbot+d29d18215e477cfbfbdd@syzkaller.appspotmail.com
---
 net/nfc/llcp_commands.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/nfc/llcp_commands.c b/net/nfc/llcp_commands.c
index 2ceefa183cee..e19fadaa9022 100644
--- a/net/nfc/llcp_commands.c
+++ b/net/nfc/llcp_commands.c

@@ -20,6 +20,7 @@
 #include <linux/init.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/sched.h>
 #include <linux/nfc.h>

 #include <net/nfc/nfc.h>

@@ -755,7 +756,8 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
                pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, MSG_DONTWAIT,
                                         frag_len + LLCP_HEADER_SIZE, &err);
                if (pdu == NULL) {
-                       pr_err("Could not allocate PDU\n");
+                       pr_err_ratelimited("Could not allocate PDU\n");
+                       cond_resched();
                        continue;
                }