[BUG] Splat during modest hazptrtorture run

[Paul E. McKenney]

Hello!

This is all new code, so the bug could be anywhere.  So you all need
to know.  ;-)

I got this from the NOPREEMPT variant of hazptr during a nominal
"--duration 60" run of torture.sh on x86 with the guest OS split across
two NUMA nodes:

[  137.765104] WARNING: kernel/rcu/hazptrtorture.c:658 at hazptr_torture_stats_print+0x25b/0x4f0, CPU#1: hazptr_torture_/136
[  137.768647] Modules linked in:
[  137.768891] CPU: 1 UID: 0 PID: 136 Comm: hazptr_torture_ Not tainted 7.3.0-rc5-00661-ga1f1e4890d1d-dirty #193 PREEMPTLAZY
[  137.769683] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-6.el9 11/05/2023
[  137.770319] RIP: 0010:hazptr_torture_stats_print+0x25b/0x4f0
[  137.770735] Code: 48 83 c4 20 e8 86 44 fe ff 41 83 fd 01 7e 23 48 c7 c6 6e 85 17 b0 48 c7 c7 db 27 17 b0 e8 6d 44 fe ff f0 ff 05 76 6e 0a 02 90 <0f> 0b 90 4c 8b 64 24 70 48 c7 c7 73 85 17 b0 e8 51 44 fe ff 48 8b
[  137.772061] RSP: 0018:ffffa738004ffdf0 EFLAGS: 00010202
[  137.772435] RAX: 0000000000000004 RBX: ffffa738004ffe08 RCX: 0000000000000027
[  137.772950] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000001
[  137.773455] RBP: ffffa738004ffe60 R08: 4000000000001236 R09: ffffffffb01727df
[  137.773980] R10: 0000000020212121 R11: 0000000020212121 R12: 0000000000000000
[  137.774479] R13: 0000000000000002 R14: 000000000002ffd6 R15: 0000000000034f03
[  137.775000] FS:  0000000000000000(0000) GS:ffff999c2e502000(0000) knlGS:0000000000000000
[  137.775578] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  137.776001] CR2: 0000000000000000 CR3: 0000000008830000 CR4: 00000000000006f0
[  137.776512] Call Trace:
[  137.776700]  <TASK>
[  137.776863]  ? __pfx_hazptr_torture_stats+0x10/0x10
[  137.777207]  hazptr_torture_stats+0x25/0x70
[  137.777508]  kthread+0xd9/0x110
[  137.777751]  ? __pfx_kthread+0x10/0x10
[  137.778017]  ret_from_fork+0x1bd/0x220
[  137.778285]  ? __pfx_kthread+0x10/0x10
[  137.778560]  ret_from_fork_asm+0x1a/0x30
[  137.778849]  </TASK>
[  137.779007] ---[ end trace 0000000000000000 ]---

I have not yet seen this during a PREEMPT run.  Line 658 of
hazptrtorture.c is the WARN_ON_ONCE() below:

	if (i > 1) {
		pr_cont("%s", "!!! ");
		atomic_inc(&n_hazptr_torture_error);
		WARN_ON_ONCE(i > 1); // Too-short grace period
	}

Any thoughts on what might be causing this?

For that matter, can anyone else reproduce this?

								Thanx, Paul