Re: [PATCH v2 6/6] x86: Fix thread_saved_pc()

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Sat, Jun 18, 2016 at 04:56:18PM -0400, Brian Gerst wrote:
> thread_saved_pc() was using a completely bogus method to get the return
> address.  Since switch_to() was previously inlined, there was no sane way
> to know where on the stack the return address was stored.  Now with the
> frame of a sleeping thread well defined, this can be implemented correctly.
> 
> Signed-off-by: Brian Gerst <brgerst@gmail.com>
> ---
>  arch/x86/include/asm/processor.h | 10 ++--------
>  arch/x86/kernel/process.c        | 10 ++++++++++
>  arch/x86/kernel/process_32.c     |  8 --------
>  3 files changed, 12 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 1e7d634..413f4f1 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -716,8 +716,6 @@ static inline void spin_lock_prefetch(const void *x)
>  	.io_bitmap_ptr		= NULL,					  \
>  }
>  
> -extern unsigned long thread_saved_pc(struct task_struct *tsk);
> -
>  /*
>   * TOP_OF_KERNEL_STACK_PADDING reserves 8 bytes on top of the ring0 stack.
>   * This is necessary to guarantee that the entire "struct pt_regs"
> @@ -767,17 +765,13 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
>  	.sp0 = TOP_OF_INIT_STACK \
>  }
>  
> -/*
> - * Return saved PC of a blocked thread.
> - * What is this good for? it will be always the scheduler or ret_from_fork.
> - */
> -#define thread_saved_pc(t)	READ_ONCE_NOCHECK(*(unsigned long *)((t)->thread.sp - 8))
> -
>  #define task_pt_regs(tsk)	((struct pt_regs *)(tsk)->thread.sp0 - 1)
>  extern unsigned long KSTK_ESP(struct task_struct *task);
>  
>  #endif /* CONFIG_X86_64 */
>  
> +extern unsigned long thread_saved_pc(struct task_struct *tsk);
> +
>  extern void start_thread(struct pt_regs *regs, unsigned long new_ip,
>  					       unsigned long new_sp);
>  
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 00ebab0..db458c4 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -513,6 +513,16 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
>  }
>  
>  /*
> + * Return saved PC of a blocked thread.
> + */
> +unsigned long thread_saved_pc(struct task_struct *tsk)
> +{
> +	struct inactive_task_frame *frame =
> +		(struct inactive_task_frame *) READ_ONCE(tsk->thread.sp);
> +	return READ_ONCE_NOCHECK(frame->ret_addr);
> +}
> +
> +/*

I would agree with the above (removed) comment:

  "What is this good for?  it will be always the scheduler or ret_from_fork."

And I'd guess the same is true for all the arches which have to
implement it.  Maybe this function (and its single call site in
sched_show_task()) should just be removed altogether?

-- 
Josh