linux-kernel - Re: [PATCH] tracing: Add trace_trigger kernel command line option

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Date:   Wed, 19 Oct 2022 20:07:45 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Tom Zanussi <zanussi@...nel.org>
Subject: Re: [PATCH] tracing: Add trace_trigger kernel command line option

On Wed, 19 Oct 2022 20:01:37 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> @@ -90,6 +90,10 @@ int unregister_tracepoint_module_notifier(struct notifier_block *nb)
>  #ifdef CONFIG_TRACEPOINTS
>  static inline void tracepoint_synchronize_unregister(void)
>  {
> +	/* Early updates do not need synchronization */
> +	if (early_boot_irqs_disabled)
> +		return;
> +
>  	synchronize_srcu(&tracepoint_srcu);
>  	synchronize_rcu();

I wonder if this check should be just moved to the RCU synchronization
code? That is, if early_boot_irqs_disabled is set, do nothing, as there
should be nothing to synchronize against.

It took me a bit to figure out why I was constantly getting:

[    1.072013] rcu: srcu_init: Setting srcu_struct sizes based on contention.
[    1.078888] ------------[ cut here ]------------
[    1.083422] Interrupts were enabled early
[    1.087399] WARNING: CPU: 0 PID: 0 at init/main.c:1061 start_kernel+0x36f/0x4e8
[    1.094656] Modules linked in:
[    1.097683] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.1.0-rc1-test+ #461
[    1.104509] Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
[    1.113411] RIP: 0010:start_kernel+0x36f/0x4e8
[    1.117817] Code: 48 27 5f e8 fd 7b 03 00 e8 55 72 e5 fe e8 6b 05 03 00 ff 15 2a f9 89 ff 0f ba e0 09 73 0e 48 c7 c7 98 26 31 a0 e8 01 5b df fe <0f> 0b c6 05 c7 2c af ff 00 e8 da 36 47 fe ff 15 14 f9 89 ff e8 56
[    1.136485] RSP: 0000:ffffffffa0603f30 EFLAGS: 00010286
[    1.141670] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[    1.148754] RDX: 0000000000000002 RSI: 00000000ffffdfff RDI: 0000000000000001
[    1.155840] RBP: 0000000000000000 R08: 0000000000000000 R09: ffffffffa0603dd8
[    1.162926] R10: 0000000000000003 R11: ffffffffa0734168 R12: 000000000000180a
[    1.170011] R13: 00000000010fecf0 R14: 0000000000000000 R15: 00000000d44e3018
[    1.177097] FS:  0000000000000000(0000) GS:ffff91909aa00000(0000) knlGS:0000000000000000
[    1.185131] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    1.190835] CR2: ffff91909edff000 CR3: 000000007760a001 CR4: 00000000000606f0
[    1.197924] Call Trace:
[    1.200345]  <TASK>


It was a bit of whack-a-mole in finding all the places that were calling
synchronize RCU.

-- Steve