linux-kernel - Re: [PATCH printk v5 03/40] printk: Prepare for SRCU console list protection

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Date:   Thu, 01 Dec 2022 22:42:25 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Nathan Chancellor <nathan@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v5 03/40] printk: Prepare for SRCU console list
 protection

Hi Nathan,

Thanks for reporting this. Patch below.

@paulmck: Please also take a look below.

On 2022-12-01, Nathan Chancellor <nathan@...nel.org> wrote:
> bad: scheduling from the idle thread!
> CPU: 0 PID: 0 Comm: swapper Not tainted 6.1.0-rc1+ #1
> Hardware name: PowerMac3,1 7400 0xc0209 PowerMac
> Call Trace:
> [c0bc1db0] [c07f07e0] dump_stack_lvl+0x34/0x50 (unreliable)
> [c0bc1dd0] [c008429c] dequeue_task_idle+0x34/0x5c
> [c0bc1df0] [c0820924] __schedule+0x56c/0x5c4
> [c0bc1e40] [c08209d0] schedule+0x54/0xfc
> [c0bc1e60] [c0826034] schedule_timeout+0x13c/0x194
> [c0bc1ea0] [c082134c] __wait_for_common+0xcc/0x1f4
> [c0bc1ee0] [c00ac8ac] synchronize_srcu+0xc8/0x12c
> [c0bc1f20] [c00a0230] unregister_console+0xc8/0x10c
> [c0bc1f40] [c009e314] register_console+0x2f4/0x390
> [c0bc1f60] [c0a17510] pmz_console_init+0x34/0x48
> [c0bc1f70] [c0a0491c] console_init+0x9c/0xf0
> [c0bc1fa0] [c09f5584] start_kernel+0x588/0x6ac
> [c0bc1ff0] [00003540] 0x3540

This config is using TINY_RCU. Its srcu_synchronize() implementation
does not check if it called before scheduling is ready. The following
patch will fix it.

@paulmck: Should it check (system_state < SYSTEM_SCHEDULING) instead
since TINY_RCU does not use @rcu_scheduler_active?

John Ogness

diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index 33adafdad261..35338e6e37e7 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -197,6 +197,9 @@ void synchronize_srcu(struct srcu_struct *ssp)
 {
 	struct rcu_synchronize rs;
 
+	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
+		return;
+
 	init_rcu_head_on_stack(&rs.head);
 	init_completion(&rs.completion);
 	call_srcu(ssp, &rs.head, wakeme_after_rcu);