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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Wed, Jun 22, 2016 at 12:27:43AM -0400, Brian Gerst wrote:
> On Mon, Jun 20, 2016 at 12:01 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > 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?
> 
> I didn't really want to stray down that path with this series.  This
> just makes it functional again.  the usefulness is still open for
> debate.

Fair enough.

Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>

-- 
Josh