linux-kernel - Re: [PATCH] riscv: keep interrupts disabled for BREAKPOINT exception

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Message-ID: <alpine.DEB.2.21.2104032020100.65251@angie.orcam.me.uk>
Date:   Sat, 3 Apr 2021 20:30:53 +0200 (CEST)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Masami Hiramatsu <mhiramat@...nel.org>
cc:     Jisheng Zhang <jszhang3@...l.ustc.edu.cn>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Guo Ren <guoren@...ux.alibaba.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: keep interrupts disabled for BREAKPOINT
 exception

On Thu, 1 Apr 2021, Masami Hiramatsu wrote:

> > > > Current riscv's kprobe handlers are run with both preemption and
> > > > interrupt enabled, this violates kprobe requirements. Fix this issue
> > > > by keeping interrupts disabled for BREAKPOINT exception.  
> > > 
> > > Not only while the breakpoint exception but also until the end of
> > > the single step (maybe you are using __BUG_INSN_32 ??) need to be
> > > disable interrupts. Can this do that?
> > > 
> > 
> > interrupt is disabled during "single step" by kprobes_save_local_irqflag()
> > and kprobes_restore_local_irqflag(). The code flow looks like: 
> > 
> > do_trap_break()   // for bp
> >   kprobe_breakpoint_handler()
> >     setup_singlestep()
> >       kprobes_restore_local_irqflag()
> > 
> > do_trap_break()  // for ss
> >   kprobe_single_step_handler()
> >     kprobes_restore_local_irqflag()
> 
> OK, thanks for the confirmation!

 Is this approach guaranteed to keep interrupt handling latency low enough 
for the system not to be negatively affected, e.g. for the purpose of NTP 
timekeeping?

  Maciej