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

Mathieu Desnoyers
2008-10-04T22:27:23Z

* Ingo Molnar (mingo@elte.hu) wrote:
>
> * 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?
>

The comment at the beginning of
http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=arch/x86/kernel/immediate.c;h=87a25db0efbd8f73d3d575e48541f2a179915da5;hb=b6148ea934f42e730571f41aa5a1a081a93995b5

explains that code modification on x86 SMP systems is not only a matter
of atomicity, but also a matter of not changing the code underneath a
running CPU which is making assumptions that it won't change underneath
without issuing a synchronizing instruction before the new code is used
by the CPU. The scheme you propose here takes care of atomicity, but
does not take care of the synchronization problem. A sync_core() would
probably be required when such modification is detected.

Also, speaking of plain atomicity, you scheme does not seem to protect
against NMIs running on a different CPU, because the non-atomic change
could race with such NMI.

Mathieu

> Ingo
>

--
Mathieu Desnoyers
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