Subject: Re: [PATCH 04/13] powerpc/rtas: avoid scheduling in rtas_os_term()

From: "Nicholas Piggin" <[email-masked]>
Date: Mon, 28 Nov 2022 12:34:55 +1000

On Sat Nov 19, 2022 at 1:07 AM AEST, Nathan Lynch wrote:
> It's unsafe to use rtas_busy_delay() to handle a busy status from
> the ibm,os-term RTAS function in rtas_os_term():
>
> Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> BUG: sleeping function called from invalid context at arch/powerpc/kernel/rtas.c:618
> in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 1, name: swapper/0
> preempt_count: 2, expected: 0
> CPU: 7 PID: 1 Comm: swapper/0 Tainted: G      D            6.0.0-rc5-02182-gf8553a572277-dirty #9
> Call Trace:
> [c000000007b8f000] [c000000001337110] dump_stack_lvl+0xb4/0x110 (unreliable)
> [c000000007b8f040] [c0000000002440e4] __might_resched+0x394/0x3c0
> [c000000007b8f0e0] [c00000000004f680] rtas_busy_delay+0x120/0x1b0
> [c000000007b8f100] [c000000000052d04] rtas_os_term+0xb8/0xf4
> [c000000007b8f180] [c0000000001150fc] pseries_panic+0x50/0x68
> [c000000007b8f1f0] [c000000000036354] ppc_panic_platform_handler+0x34/0x50
> [c000000007b8f210] [c0000000002303c4] notifier_call_chain+0xd4/0x1c0
> [c000000007b8f2b0] [c0000000002306cc] atomic_notifier_call_chain+0xac/0x1c0
> [c000000007b8f2f0] [c0000000001d62b8] panic+0x228/0x4d0
> [c000000007b8f390] [c0000000001e573c] do_exit+0x140c/0x1420
> [c000000007b8f480] [c0000000001e586c] make_task_dead+0xdc/0x200
>
> Use rtas_busy_delay_time() instead, which signals without side effects
> whether to attempt the ibm,os-term RTAS call again.

rtas_busy_delay should probably be renamed to rtas_busy_sleep, to make
that self-documenting that it can schedule. You could then add a
rtas_busy_delay which doesn't sleep, which a few other places could
use...

But that's a bigger chance and there is precedent for using this call
this way, so looks okay to me. Maybe you could open-code an mdelay
though, although I guess firmware should be tolerant of calling it in
a loop.

Reviewed-by: Nicholas Piggin <[email-masked]>

>
> Signed-off-by: Nathan Lynch <[email-masked]>
> ---
>  arch/powerpc/kernel/rtas.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
> index 81e4996012b7..51f0508593a7 100644
> --- a/arch/powerpc/kernel/rtas.c
> +++ b/arch/powerpc/kernel/rtas.c
> @@ -965,10 +965,15 @@ void rtas_os_term(char *str)
>  
>  	snprintf(rtas_os_term_buf, 2048, "OS panic: %s", str);
>  
> +	/*
> +	 * Keep calling as long as RTAS returns a "try again" status,
> +	 * but don't use rtas_busy_delay(), which potentially
> +	 * schedules.
> +	 */
>  	do {
>  		status = rtas_call(ibm_os_term_token, 1, 1, NULL,
>  				   __pa(rtas_os_term_buf));
> -	} while (rtas_busy_delay(status));
> +	} while (rtas_busy_delay_time(status));
>  
>  	if (status != 0)
>  		printk(KERN_EMERG "ibm,os-term call failed %d\n", status);
> -- 
> 2.37.1