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

[Masami Hiramatsu]

Hi Zong,

On Fri, 3 Apr 2020 17:04:51 +0800
Zong Li <zong.li@xxxxxxxxxx> 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...)

Thank you,
-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>