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From:   John Ogness <john.ogness@linutronix.de>
To:     paulmck@kernel.org
Cc:     Frederic Weisbecker <frederic@kernel.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        rostedt@goodmis.org, tglx@linutronix.de, pmladek@suse.com
Subject: Re: [PATCH v2 rcu 0/8] NMI-safe SRCU reader API
In-Reply-To: <20221024134727.GV5600@paulmck-ThinkPad-P17-Gen-1>
References: <87ilkh0y52.fsf@jogness.linutronix.de>
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Date:   Thu, 27 Oct 2022 11:37:46 +0206
Message-ID: <874jvpehod.fsf@jogness.linutronix.de>
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Hi Paul,

I am running some tests using srcunmisafe.2022.10.21a and I am hitting
the WARN_ONCE in srcu_check_nmi_safety():

[    1.836703][    T1] rcu: Hierarchical SRCU implementation.
[    1.836707][    T1] rcu:     Max phase no-delay instances is 1000.
[    1.836844][   T15] ------------[ cut here ]------------
[    1.836846][   T15] CPU 0 old state 1 new state 2
[    1.836885][   T15] WARNING: CPU: 0 PID: 15 at kernel/rcu/srcutree.c:652 srcu_check_nmi_safety+0x79/0x90
[    1.836897][   T15] Modules linked in:
[    1.836903][   T15] CPU: 0 PID: 15 Comm: pr/bkl Not tainted 6.1.0-rc1+ #9
[    1.836909][   T15] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
[    1.836912][   T15] RIP: 0010:srcu_check_nmi_safety+0x79/0x90
[    1.836919][   T15] Code: d3 80 3d 9f 76 d3 01 00 75 e5 55 8b b0 c8 01 00 00 44 89 c1 48 c7 c7 d0 1f 87 82 c6 05 850
[    1.836923][   T15] RSP: 0000:ffffc90000083e98 EFLAGS: 00010282
[    1.836929][   T15] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[    1.836933][   T15] RDX: 0000000000000001 RSI: 0000000000000000 RDI: 00000000ffffffff
[    1.836936][   T15] RBP: ffffc90000083e98 R08: 0000000000000000 R09: 0000000000000001
[    1.836940][   T15] R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
[    1.836943][   T15] R13: ffffc90000013d70 R14: ffff888004073900 R15: 0000000000000000
[    1.836946][   T15] FS:  0000000000000000(0000) GS:ffff888019600000(0000) knlGS:0000000000000000
[    1.836951][   T15] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    1.836954][   T15] CR2: ffff888003c01000 CR3: 0000000002c22001 CR4: 0000000000370ef0
[    1.836962][   T15] Call Trace:
[    1.836964][   T15]  <TASK>
[    1.836970][   T15]  console_bkl_kthread_func+0x27a/0x590
[    1.836981][   T15]  ? _raw_spin_unlock_irqrestore+0x3c/0x60
[    1.836998][   T15]  ? console_fill_outbuf+0x210/0x210
[    1.837003][   T15]  kthread+0x108/0x130
[    1.837012][   T15]  ? kthread_complete_and_exit+0x20/0x20
[    1.837025][   T15]  ret_from_fork+0x1f/0x30
[    1.837059][   T15]  </TASK>
[    1.837062][   T15] irq event stamp: 71
[    1.837065][   T15] hardirqs last  enabled at (73): [<ffffffff81106f99>] vprintk_store+0x1b9/0x5e0
[    1.837070][   T15] hardirqs last disabled at (74): [<ffffffff811071fb>] vprintk_store+0x41b/0x5e0
[    1.837075][   T15] softirqs last  enabled at (0): [<ffffffff8107ce22>] copy_process+0x952/0x1dd0
[    1.837081][   T15] softirqs last disabled at (0): [<0000000000000000>] 0x0
[    1.837085][   T15] ---[ end trace 0000000000000000 ]---
[    1.945054][   T12] Callback from call_rcu_tasks_rude() invoked.

My code is calling srcu_read_lock_nmisafe() from task context, in a
dedicated kthread. I am using DEFINE_STATIC_SRCU() to define/initialize
the srcu struct.

What does the warning imply?

John Ogness