Subject: Re: [PATCH] powerpc: Set paca->data_offset = 0 for boot cpu

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Fri, 21 Sep 2012 14:07:15 +0530

Michael Ellerman <[email-masked]> writes:

> In commit 407821a we assigned a poison value to the paca->data_offset.
>
> Unfortunately with CONFIG_LOCK_STAT=y lockdep will read & write to percpu
> data very early in boot, prior to us initialising the percpu areas,
> leading to a crash.
>
> We have been getting away with this because the data_offset was previously
> set to zero. This causes lockdep to read & write to the initial copy of
> the percpu variables, which are discarded later in boot.
>
> Although that is "fishy", it does work, and for lock statistics it is no
> big deal to discard the counts from early boot.
>
> So set the paca->data_offset = 0 for the boot cpu paca only.
>
> Reported-by: Aneesh Kumar K.V <[email-masked]>
> Signed-off-by: Michael Ellerman <[email-masked]>

Tested-by: Aneesh Kumar K.V <[email-masked]>

> ---
>  arch/powerpc/kernel/setup_64.c |    2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
> index 389bd4f..efb6a41 100644
> --- a/arch/powerpc/kernel/setup_64.c
> +++ b/arch/powerpc/kernel/setup_64.c
> @@ -208,6 +208,8 @@ void __init early_setup(unsigned long dt_ptr)
>
>  	/* Fix up paca fields required for the boot cpu */
>  	get_paca()->cpu_start = 1;
> +	/* Allow percpu accesses to "work" until we setup percpu data */
> +	get_paca()->data_offset = 0;
>
>  	/* Probe the machine type */
>  	probe_machine();
> -- 
> 1.7.9.5