Re: INFO: rcu detected stall in smp_call_function

[peterz]

On Wed, Jul 29, 2020 at 01:44:15AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    6ba1b005 Merge tag 'asm-generic-fixes-5.8' of git://git.ke..

Bit useless, having git://git.ke... there, that doesn't really narrow
things down.

> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=14da5522900000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=812bbfcb6ae2cd60
> dashboard link: https://syzkaller.appspot.com/bug?extid=cb3b69ae80afd6535b0e
> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+cb3b69ae80afd6535b0e@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> rcu: 	Tasks blocked on level-0 rcu_node (CPUs 0-1):
> ------------[ cut here ]------------
> IRQs not enabled as expected
> WARNING: CPU: 0 PID: 32297 at kernel/sched/core.c:2701 try_invoke_on_locked_down_task+0x18b/0x320 kernel/sched/core.c:2701
> Kernel panic - not syncing: panic_on_warn set ...
> CPU: 0 PID: 32297 Comm: syz-executor.2 Not tainted 5.8.0-rc7-syzkaller #0
> 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+0x1f0/0x31e lib/dump_stack.c:118
>  panic+0x264/0x7a0 kernel/panic.c:231
>  __warn+0x227/0x250 kernel/panic.c:600
>  report_bug+0x1b1/0x2e0 lib/bug.c:198
>  handle_bug+0x42/0x80 arch/x86/kernel/traps.c:235
>  exc_invalid_op+0x16/0x40 arch/x86/kernel/traps.c:255
>  asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:540
> RIP: 0010:try_invoke_on_locked_down_task+0x18b/0x320 kernel/sched/core.c:2701
> Code: 48 89 df e8 f7 35 09 00 4c 89 f7 e8 df b5 cf 06 e9 b5 00 00 00 c6 05 34 82 38 08 01 48 c7 c7 8c d7 07 89 31 c0 e8 a5 a9 f5 ff <0f> 0b e9 15 ff ff ff 48 c7 c1 30 71 8d 89 80 e1 07 80 c1 03 38 c1
> RSP: 0018:ffffc90000007c50 EFLAGS: 00010046
> RAX: 1aaa08be6903c500 RBX: ffff888085d16ac8 RCX: ffff888085d16240
> RDX: 0000000000010004 RSI: 0000000000010004 RDI: 0000000000000000
> RBP: ffff888085d16b0c R08: ffffffff815dd389 R09: ffffed1015d041c3
> R10: ffffed1015d041c3 R11: 0000000000000000 R12: 0000000000000000
> R13: ffff8880a8bac140 R14: ffff8880a8bac4c0 R15: dffffc0000000000
>  rcu_print_task_stall kernel/rcu/tree_stall.h:269 [inline]
>  print_other_cpu_stall kernel/rcu/tree_stall.h:477 [inline]

Ha, that calls it with IRQs already disabled,

So I'm thinking we want something like so?

---

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2142c6767682..3182caf14844 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2694,12 +2694,11 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
  */
 bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg)
 {
-	bool ret = false;
 	struct rq_flags rf;
+	bool ret = false;
 	struct rq *rq;
 
-	lockdep_assert_irqs_enabled();
-	raw_spin_lock_irq(&p->pi_lock);
+	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
 	if (p->on_rq) {
 		rq = __task_rq_lock(p, &rf);
 		if (task_rq(p) == rq)
@@ -2716,7 +2715,7 @@ bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct t
 				ret = func(p, arg);
 		}
 	}
-	raw_spin_unlock_irq(&p->pi_lock);
+	raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
 	return ret;
 }