[Crash-utility] [PATCH] Fix x86 initialization for {hard, soft}irq_ctx

[Dave Anderson]

----- Original Message -----
> 
> ----- Original Message -----
> >
> > Current code is setting {hard,soft}irq_ctx[] to (irq_ctx **), because
> > per_cpu symbol itself is pointer of specified type (irq_ctx *).
> >
> > But, I wonder how this works in past, the code is expecting
> > {hard,soft}_ctx[] are (irq_ctx *). This fixes by deref per_cpu in
> > initialization, and set expected pointers.
> >
> > Tested on i386 v3.10.
> >
> 
> You are correct -- apparently it never did work.
> 
> The reason why I never noticed with RHEL kernels is that up until
> linux-2.6.29,
> the IRQ stacks were declared as a static NR_CPUS-bounded array:
>   
>   #ifdef CONFIG_4KSTACKS
>   /*
>    * per-CPU IRQ handling contexts (thread information and stack)
>    */
>   union irq_ctx {
>           struct thread_info      tinfo;
>           u32                     stack[THREAD_SIZE/sizeof(u32)];
>   };
>   
>   static union irq_ctx *hardirq_ctx[NR_CPUS] __read_mostly;
>   static union irq_ctx *softirq_ctx[NR_CPUS] __read_mostly;
>   
>   static char softirq_stack[NR_CPUS * THREAD_SIZE] __page_aligned_bss;
>   static char hardirq_stack[NR_CPUS * THREAD_SIZE] __page_aligned_bss;
>   
>   
> Then in linux-2.6.30, they were changed to be per-cpu variables:
>   
>   #ifdef CONFIG_4KSTACKS
>   /*
>    * per-CPU IRQ handling contexts (thread information and stack)
>    */
>   union irq_ctx {
>           struct thread_info      tinfo;
>           u32                     stack[THREAD_SIZE/sizeof(u32)];
>   } __attribute__((aligned(PAGE_SIZE)));
>   
>   static DEFINE_PER_CPU(union irq_ctx *, hardirq_ctx);
>   static DEFINE_PER_CPU(union irq_ctx *, softirq_ctx);
>   
>   static DEFINE_PER_CPU_PAGE_ALIGNED(union irq_ctx, hardirq_stack);
>   static DEFINE_PER_CPU_PAGE_ALIGNED(union irq_ctx, softirq_stack);
>   
> We never ran into the bug in 32-bit x86 RHEL6 (2.6.32-based) kernels because
> we did not configure CONFIG_4KSTACKS.
> 
> Later on, the CONFIG_4KSTACKS restriction looks like it was removed, but we
> stopped supporting 32-bit x86 in RHEL7.  And nobody ever reported it from any
> other x86 kernel sources.
> 
> I also note that in linux-3.15 the data structure name and variables were
> changed to:
>  
>   struct irq_stack {
>           u32                     stack[THREAD_SIZE/sizeof(u32)];
>   } __aligned(THREAD_SIZE);
> 
>   DECLARE_PER_CPU(struct irq_stack *, hardirq_stack);
>   DECLARE_PER_CPU(struct irq_stack *, softirq_stack);
> 
> So your patch would need more work for 3.15 and later kernels.
> 
> Thanks,
>   Dave

Hi Ogawa,

Please review the attached update to your patch, which adds support for 
the irq_ctx-to-irq_stack transition.

Thanks,
  Dave
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