[PATCH] Prevent process migration during vfp_init()

[Will Deacon]

On Fri, May 04, 2012 at 09:28:15PM +0100, Hyungwoo Yang wrote:
> Hello,

Hi,

Some comments inline and a proposed v2.

> From f96fc79d508235706462336239eb30d66e2e6c0b Mon Sep 17 00:00:00 2001
> From: Hyungwoo Yang <hyungwooy at nvidia.com>
> Date: Fri, 4 May 2012 11:22:59 -0700
> Subject: [PATCH] System crashes if there is process migration during
> vfp_init() call.
> 
> During vfp_init(), if a process which called vfp_enable() is migrated just
> after the call, then the process executing the rest of code will access
> a VFP unit which is not ENABLED and also smp_call_function() will not work
> as it is expected.
> 
> This patch prevents accessing VFP unit disabled by preventing migration
> and also replaces smp_call_function() with on_each_cpu() to make sure that
> no VFP remains disabled.
> 
> Signed-off-by: Hyungwoo Yang <hyungwooy at nvidia.com>
> 
> diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
> index bc683b8..6f33e4d 100644
> --- a/arch/arm/vfp/vfpmodule.c
> +++ b/arch/arm/vfp/vfpmodule.c
> @@ -655,7 +655,9 @@ static int __init vfp_init(void)
>  {
>  	unsigned int vfpsid;
>  	unsigned int cpu_arch = cpu_architecture();
> -
> +#ifdef CONFIG_SMP
> +	preempt_disable();
> +#endif

I don't think it's worth having the SMP guards for this.

>  	if (cpu_arch >= CPU_ARCH_ARMv6)
>  		vfp_enable(NULL);

I think we can avoid this redundant enable if we move some code around (see
below).

> @@ -667,7 +669,9 @@ static int __init vfp_init(void)
>  	vfp_vector = vfp_testing_entry;
>  	barrier();
>  	vfpsid = fmrx(FPSID);
> -	barrier();
> +#ifdef CONFIG_SMP
> +	preempt_enable();
> +#endif

If we're not running a PREEMPT kernel, you just removed a compiler barrier.

How about something like the following instead? I've also added a BUG_ON to
vfp_enable to stop this catching us out in future:

diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
index bc683b8..8e08983 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)
@@ -657,18 +661,20 @@ 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.
         * The handler is already setup to just log calls, so
         * we just need to read the VFPSID register.
         */
+       preempt_disable();
        vfp_vector = vfp_testing_entry;
        barrier();
        vfpsid = fmrx(FPSID);
        barrier();
        vfp_vector = vfp_null_entry;
+       preempt_enable();
 
        printk(KERN_INFO "VFP support v0.3: ");
        if (VFP_arch)
@@ -678,8 +684,6 @@ 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,

Will