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

From: Andrew Donnellan <ajd@linux.ibm.com>
To: Nathan Lynch <nathanl@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org
Cc: tyreld@linux.ibm.com, nnac123@linux.ibm.com,
	ldufour@linux.ibm.com, npiggin@gmail.com
Subject: Re: [PATCH 03/13] powerpc/rtas: avoid device tree lookups in rtas_os_term()
Date: Tue, 22 Nov 2022 14:03:59 +1100	[thread overview]
Message-ID: <2bb6286d131fe34b4358a962f708ee0a9b85e166.camel@linux.ibm.com> (raw)
In-Reply-To: <20221118150751.469393-4-nathanl@linux.ibm.com>

On Fri, 2022-11-18 at 09:07 -0600, 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.
> 
> 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.
> 
> Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>

This seems sensible, minor comment below.

Reviewed-by: Andrew Donnellan <ajd@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;

s32 and int are obviously identical, but rtas_token is declared using
int, as are the other variables where we cache various tokens.

> +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
>          */

-- 
Andrew Donnellan    OzLabs, ADL Canberra
ajd@linux.ibm.com   IBM Australia Limited