linux-kernel - Re: [PATCH v3 8/9] riscv: introduce interfaces to patch kernel code

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Date:   Sat, 4 Apr 2020 20:12:24 +0800
From:   Zong Li <zong.li@...ive.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 8/9] riscv: introduce interfaces to patch kernel code

On Sat, Apr 4, 2020 at 11:14 AM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> Hi Zong,
>
> On Fri, 3 Apr 2020 17:04:51 +0800
> Zong Li <zong.li@...ive.com> wrote:
>
> > > > > > +{
> > > > > > +     void *waddr = addr;
> > > > > > +     bool across_pages = (((uintptr_t) addr & ~PAGE_MASK) + len) > PAGE_SIZE;
> > > > > > +     unsigned long flags = 0;
> > > > > > +     int ret;
> > > > > > +
> > > > > > +     raw_spin_lock_irqsave(&patch_lock, flags);
> > > > >
> > > > > This looks a bit odd since stop_machine() is protected by its own mutex,
> > > > > and also the irq is already disabled here.
> > > >
> > > > We need it because we don't always enter the riscv_patch_text_nosync()
> > > > through stop_machine mechanism. If we call the
> > > > riscv_patch_text_nosync() directly, we need a lock to protect the
> > > > page.
> > >
> > > Oh, OK, but it leads another question. Is that safe to patch the
> > > text without sync? Would you use it for UP system?
> > > I think it is better to clarify "in what case user can call _nosync()"
> > > and add a comment on it.
> >
> > The ftrace is one of the cases, as documentation of ftrace said, when
> > dynamic ftrace is initialized, it calls kstop_machine to make the
> > machine act like a uniprocessor so that it can freely modify code
> > without worrying about other processors executing that same code. So
> > the ftrace called the _nosync interface here directly.
>
> Hmm, even though, since it already running under kstop_machine(), no
> other thread will run.
> Could you consider to use text_mutex instead of that? The text_mutex
> is already widely used in x86 and kernel/kprobes.c etc.
>
> (Hmm, it seems except for x86, alternative code don't care about
>  racing...)
>

Yes, text_mutex seems to be great. I'll change to use text_mutex in
the next version if it works fine after testing. Thanks.

> Thank you,
> --
> Masami Hiramatsu <mhiramat@...nel.org>