[PATCH] Prevent process migration during vfp_init()

[Hyungwoo Yang]

Sure, please go ahead.

On Tue, May 8, 2012 at 11:45 AM, Will Deacon <will.deacon at arm.com> wrote:
> On Tue, May 08, 2012 at 07:28:08PM +0100, Hyungwoo Yang wrote:
>> Yes, I've already tested without preemption, it seems it is working fine.
>
> Cracking, thanks for confirming that.
>
>> for the failure, it is simple. on my A9 quad, due to heavy loads
>> during "late initcall", sometimes process migration happens just after
>> vfp_enable() but before smp_call_function(). it means there can be a
>> VFP disabled but accessed since smp_call_function() is for the rest of
>> cores not the core running it.
>>
>> For example, if enable_vfp() is executed on core0 but
>> smp_call_function() is excuted on core1 due to migration, then VFP on
>> core1 is not enabled. it will be enabled next time when the core1
>> online again.
>
> Yup, your initial explanation was clear enough, I just wanted to check that
> this did indeed solve your testcase given that I've been lucky enough not to
> hit the issue on my boards.
>
> Can I add your signed-off-by to the following please?
>
> Will
>
>
> Subject: [PATCH] ARM: vfp: ensure preemption is disabled when enabling VFP access
>
> The vfp_enable function enables access to the VFP co-processor register
> space (cp10 and cp11) on the current CPU and must be called with
> preemption disabled. Unfortunately, the vfp_init late initcall does not
> disable preemption and can lead to an oops during boot if thread
> migration occurs at the wrong time and we end up attempting to access
> the FPSID on a CPU with VFP access disabled.
>
> This patch fixes the initcall to call vfp_enable from a non-preemptible
> context on each CPU and adds a BUG_ON(preemptible) to ensure that any
> similar problems are easily spotted in the future.
>
> Reported-by: Hyungwoo Yang <hwoo.yang at gmail.com>
> Signed-off-by: Will Deacon <will.deacon at arm.com>
> ---
>  arch/arm/vfp/vfpmodule.c |   10 ++++++----
>  1 files changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
> index bc683b8..c5767b5 100644
> --- a/arch/arm/vfp/vfpmodule.c
> +++ b/arch/arm/vfp/vfpmodule.c
> @@ -11,6 +11,7 @@
>  #include <linux/types.h>
>  #include <linux/cpu.h>
>  #include <linux/cpu_pm.h>
> +#include <linux/hardirq.h>
>  #include <linux/kernel.h>
>  #include <linux/notifier.h>
>  #include <linux/signal.h>
> @@ -432,7 +433,10 @@ void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs)
>
>  static void vfp_enable(void *unused)
>  {
> -       u32 access = get_copro_access();
> +       u32 access;
> +
> +       BUG_ON(preemptible());
> +       access = get_copro_access();
>
>        /*
>         * Enable full access to VFP (cp10 and cp11)
> @@ -657,7 +661,7 @@ static int __init vfp_init(void)
>        unsigned int cpu_arch = cpu_architecture();
>
>        if (cpu_arch >= CPU_ARCH_ARMv6)
> -               vfp_enable(NULL);
> +               on_each_cpu(vfp_enable, NULL, 1);
>
>        /*
>         * First check that there is a VFP that we can use.
> @@ -678,8 +682,6 @@ static int __init vfp_init(void)
>        } else {
>                hotcpu_notifier(vfp_hotplug, 0);
>
> -               smp_call_function(vfp_enable, NULL, 1);
> -
>                VFP_arch = (vfpsid & FPSID_ARCH_MASK) >> FPSID_ARCH_BIT;  /* Extract the architecture version */
>                printk("implementor %02x architecture %d part %02x variant %x rev %x\n",
>                        (vfpsid & FPSID_IMPLEMENTER_MASK) >> FPSID_IMPLEMENTER_BIT,
> --
> 1.7.4.1