Re: [PATCH] arm64: make irq_stack_ptr more robust

[Shi, Yang]

On 2/12/2016 9:38 AM, Shi, Yang wrote:
> On 2/12/2016 5:47 AM, James Morse wrote:
> > Hi!
> >
> > On 11/02/16 21:53, Yang Shi wrote:
> > > Switching between stacks is only valid if we are tracing ourselves
> > > while on the
> > > irq_stack, so it is only valid when in current and non-preemptible
> > > context,
> > > otherwise is is just zeroed off.
> >
> > Given it was picked up with CONFIG_DEBUG_PREEMPT:
> >
> > Fixes: 132cd887b5c5 ("arm64: Modify stack trace and dump for use with
> > irq_stack")
>
> Wii add in v2.
>
> > > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx>
> > > ---
> > >   arch/arm64/kernel/stacktrace.c | 13 ++++++-------
> > >   arch/arm64/kernel/traps.c      | 11 ++++++++++-
> > >   2 files changed, 16 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/arm64/kernel/stacktrace.c
> > > b/arch/arm64/kernel/stacktrace.c
> > > index 12a18cb..d9751a4 100644
> > > --- a/arch/arm64/kernel/stacktrace.c
> > > +++ b/arch/arm64/kernel/stacktrace.c
> > > @@ -44,14 +44,13 @@ int notrace unwind_frame(struct task_struct *tsk,
> > > struct stackframe *frame)
> > >       unsigned long irq_stack_ptr;
> > >
> > >       /*
> > > -     * Use raw_smp_processor_id() to avoid false-positives from
> > > -     * CONFIG_DEBUG_PREEMPT. get_wchan() calls unwind_frame() on
> > > sleeping
> > > -     * task stacks, we can be pre-empted in this case, so
> > > -     * {raw_,}smp_processor_id() may give us the wrong value. Sleeping
> > > -     * tasks can't ever be on an interrupt stack, so regardless of cpu,
> > > -     * the checks will always fail.
> > > +     * Switching between stacks is valid when tracing current and in
> > > +     * non-preemptible context.
> > >        */
> > > -    irq_stack_ptr = IRQ_STACK_PTR(raw_smp_processor_id());
> > > +    if (tsk == current && !preemptible())
> > > +        irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
> > > +    else
> > > +        irq_stack_ptr = 0;
> > >
> > >       low  = frame->sp;
> > >       /* irq stacks are not THREAD_SIZE aligned */
> > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > > index cbedd72..7d8db3a 100644
> > > --- a/arch/arm64/kernel/traps.c
> > > +++ b/arch/arm64/kernel/traps.c
> > > @@ -146,9 +146,18 @@ static void dump_instr(const char *lvl, struct
> > > pt_regs *regs)
> > >   static void dump_backtrace(struct pt_regs *regs, struct task_struct
> > > *tsk)
> > >   {
> > >       struct stackframe frame;
> > > -    unsigned long irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
> > > +    unsigned long irq_stack_ptr;
> > >       int skip;
> > >
> > > +    /*
> > > +     * Switching between  stacks is valid when tracing current and in
> >
> > Nit: Two spaces: "between[ ][ ]stacks"
>
> Will fix in v2.
>
> > > +     * non-preemptible context.
> > > +     */
> > > +    if (tsk == current && !preemptible())
> > > +        irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
> > > +    else
> > > +        irq_stack_ptr = 0;
> > > +
> > >       pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
> > >
> > >       if (!tsk)
> >
> > Neither file includes 'linux/preempt.h' for the definition of
> > preemptible().
> > (I can't talk: I should have included smp.h for smp_processor_id())
>
> I tried to build the kernel with preempt and without preempt, both
> works. And, I saw arch/arm64/include/asm/Kbuild has:
>
> generic-y += preempt.h
>
> So, it sounds preempt.h has been included by default.

In addition, linux/sched.h, which is included by both traps.c and 
stacktrace.c, includes preempt.h already.

Yang

> Thanks,
> Yang
>
> > Acked-by: James Morse <james.morse@xxxxxxx>
> > Tested-by: James Morse <james.morse@xxxxxxx>
> >
> >
> > Thanks!
> >
> > James