Re: [PATCH] arm64: reenable interrupt when handling ptrace breakpoint

[Shi, Yang]

On 1/13/2016 2:26 AM, Will Deacon wrote:
> On Tue, Jan 12, 2016 at 11:59:54AM -0800, Shi, Yang wrote:
> > On 12/21/2015 9:00 AM, Will Deacon wrote:
> > > On Mon, Dec 21, 2015 at 05:51:22PM +0100, Thomas Gleixner wrote:
> > > > On Mon, 21 Dec 2015, Will Deacon wrote:
> > > > > +static void send_user_sigtrap(int si_code)
> > > > > +{
> > > > > +	struct pt_regs *regs = current_pt_regs();
> > > > > +	siginfo_t info = {
> > > > > +		.si_signo	= SIGTRAP,
> > > > > +		.si_errno	= 0,
> > > > > +		.si_code	= si_code,
> > > > > +		.si_addr	= (void __user *)instruction_pointer(regs),
> > > > > +	};
> > > > > +
> > > > > +	if (WARN_ON(!user_mode(regs)))
> > > > > +		return;
> > > > > +
> > > > > +	preempt_disable();
> > > >
> > > > That doesn't work on RT either. force_sig_info() takes task->sighand->siglock,
> > > > which is a 'sleeping' spinlock on RT.
> > >
> > > Ah, I missed that :/
> > >
> > > > Why would we need to disable preemption here at all? What's the problem of
> > > > being preempted or even migrated?
> > >
> > > There *might* not be a problem, I'm just really nervous about changing
> > > the behaviour on the debug path and subtly changing how ptrace behaves.
> > >
> > > My worry was that you could somehow get back into the tracer, and it
> > > could remove a software breakpoint in the knowledge that it wouldn't
> > > see any future (spurious) SIGTRAPs for that location.
> > >
> > > Without a concrete example, however, I guess I'll bite the bullet and
> > > enable irqs across the call to force_sig_info, since there is clearly a
> > > real issue here on RT.
> >
> > This might be buried in email storm during the holiday. Just want to double
> > check the status. I'm supposed there is no objection for getting it merged
> > in upstream?
>
> Sorry, when you replied with:
>
> > I think we could just extend the "signal delay send" approach from x86-64
> > to arm64, which is currently used by x86-64 on -rt kernel only.
>
> I understood that you were going to fix -rt, so I dropped this pending
> anything more from you.
>
> What's the plan?

Sorry for the confusion. The "signal delay send" approach used by x86-64 
-rt should be not necessary for arm64 right now. Reenabling interrupt is 
still the preferred approach.

Since x86-64 has per-CPU IST exception stack, so preemption has to be 
disabled all the time. However, it is not applicable to other 
architectures for now, including arm64.

Thanks,
Yang

> Will