Re: [PATCH 0/3] ring-buffer: less locking and only disable preemption

Ingo Molnar
2008-10-04T17:41:44Z

* Ingo Molnar <mingo@elte.hu> wrote:

> * Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > The dynamic function tracer is another issue. The problem with NMIs
> > has nothing to do with locking, or corrupting the buffers. It has to
> > do with the dynamic code modification. Whenever we modify code, we
> > must guarantee that it will not be executed on another CPU.
> >
> > Kstop_machine serves this purpose rather well. We can modify code
> > without worrying it will be executed on another CPU, except for NMIs.
> > The problem now comes where an NMI can come in and execute the code
> > being modified. That's why I put in all the notrace, lines. But it
> > gets difficult because of nmi_notifier can call all over the kernel.
> > Perhaps, we can simply disable the nmi-notifier when we are doing the
> > kstop_machine call?
>
> that would definitely be one way to reduce the cross section, but not
> enough i'm afraid. For example in the nmi_watchdog=2 case we call into
> various lapic functions and paravirt lapic handlers which makes it all
> spread to 3-4 paravirtualization flavors ...
>
> sched_clock()'s notrace aspects were pretty manageable, but this in
> its current form is not.

there's a relatively simple method that would solve all these
impact-size problems.

We cannot stop NMIs (and MCEs, etc.), but we can make kernel code
modifications atomic, by adding the following thin layer ontop of it:

#define MAX_CODE_SIZE 10

int redo_len;
u8 *redo_vaddr;

u8 redo_buffer[MAX_CODE_SIZE];

atomic_t __read_mostly redo_pending;

and use it in do_nmi():

if (unlikely(atomic_read(&redo_pending)))
modify_code_redo();

i.e. when we modify code, we first fill in the redo_buffer[], redo_vaddr
and redo_len[], then we set redo_pending flag. Then we modify the kernel
code, and clear the redo_pending flag.

If an NMI (or MCE) handler intervenes, it will notice the pending
'transaction' and will copy redo_buffer[] to the (redo_vaddr,len)
location and will continue.

So as far as non-maskable contexts are concerned, kernel code patching
becomes an atomic operation. do_nmi() has to be marked notrace but
that's all and easy to maintain.

Hm?

Ingo