[PATCH 03/13] powerpc/rtas: avoid device tree lookups in rtas_os_term()

[Nicholas Piggin]

On Sat Nov 19, 2022 at 1:07 AM AEST, Nathan Lynch wrote:
> rtas_os_term() is called during panic. Its behavior depends on a
> couple of conditions in the /rtas node of the device tree, the
> traversal of which entails locking and local IRQ state changes. If the
> kernel panics while devtree_lock is held, rtas_os_term() as currently
> written could hang.

Nice.

>
> Instead of discovering the relevant characteristics at panic time,
> cache them in file-static variables at boot. Note the lookup for
> "ibm,extended-os-term" is converted to of_property_read_bool() since
> it is a boolean property, not a RTAS function token.

Small nit, but you could do that at the query site unless you
were going to start using ibm,os-term without the extended
capability.

Reviewed-by: Nicholas Piggin <npiggin at gmail.com>

>
> Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
> ---
>  arch/powerpc/kernel/rtas.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
> index c12dd5ed5e00..81e4996012b7 100644
> --- a/arch/powerpc/kernel/rtas.c
> +++ b/arch/powerpc/kernel/rtas.c
> @@ -947,6 +947,8 @@ void __noreturn rtas_halt(void)
>  
>  /* Must be in the RMO region, so we place it here */
>  static char rtas_os_term_buf[2048];
> +static s32 ibm_os_term_token = RTAS_UNKNOWN_SERVICE;
> +static bool ibm_extended_os_term;
>  
>  void rtas_os_term(char *str)
>  {
> @@ -958,14 +960,13 @@ void rtas_os_term(char *str)
>  	 * this property may terminate the partition which we want to avoid
>  	 * since it interferes with panic_timeout.
>  	 */
> -	if (RTAS_UNKNOWN_SERVICE == rtas_token("ibm,os-term") ||
> -	    RTAS_UNKNOWN_SERVICE == rtas_token("ibm,extended-os-term"))
> +	if (ibm_os_term_token == RTAS_UNKNOWN_SERVICE || !ibm_extended_os_term)
>  		return;
>  
>  	snprintf(rtas_os_term_buf, 2048, "OS panic: %s", str);
>  
>  	do {
> -		status = rtas_call(rtas_token("ibm,os-term"), 1, 1, NULL,
> +		status = rtas_call(ibm_os_term_token, 1, 1, NULL,
>  				   __pa(rtas_os_term_buf));
>  	} while (rtas_busy_delay(status));
>  
> @@ -1335,6 +1336,13 @@ void __init rtas_initialize(void)
>  	no_entry = of_property_read_u32(rtas.dev, "linux,rtas-entry", &entry);
>  	rtas.entry = no_entry ? rtas.base : entry;
>  
> +	/*
> +	 * Discover these now to avoid device tree lookups in the
> +	 * panic path.
> +	 */
> +	ibm_os_term_token = rtas_token("ibm,os-term");
> +	ibm_extended_os_term = of_property_read_bool(rtas.dev, "ibm,extended-os-term");
> +
>  	/* If RTAS was found, allocate the RMO buffer for it and look for
>  	 * the stop-self token if any
>  	 */
> -- 
> 2.37.1