linux-kernel - Re: [PATCH v2 6/6] x86: Fix thread_saved

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Date:	Wed, 22 Jun 2016 00:27:43 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 6/6] x86: Fix thread_saved_pc()

On Mon, Jun 20, 2016 at 12:01 PM, Josh Poimboeuf <jpoimboe@...hat.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@...il.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.

--
Brian Gerst