Re: new text patching for review

[Andi Kleen <andi@firstfloor.org>]

On Thu, Jul 19, 2007 at 07:49:12PM -0400, Mathieu Desnoyers wrote:
> * Andi Kleen (andi@firstfloor.org) wrote:
> > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> writes:
> > 
> > > * Andi Kleen (ak@suse.de) wrote:
> > > > 
> > > > > Ewwwwwwwwwww.... you plan to run this in SMP ? So you actually go byte
> > > > > by byte changing pieces of instructions non atomically and doing
> > > > > non-Intel's errata friendly XMC. You are really looking for trouble
> > > > > there :) Two distinct errors can occur: 
> > > > 
> > > > In this case it is ok because this only happens when transitioning
> > > > from 1 CPU to 2 CPUs or vice versa and in both cases the other CPUs
> > > > are essentially stopped.
> > > > 
> > > 
> > > I agree that it's ok with SMP, but another problem arises: it's not only
> > > a matter of being protected from SMP access, but also a matter of
> > > reentrancy wrt interrupt handlers.
> > > 
> > > i.e.: if, as we are patching nops non atomically, we have a non maskable
> > > interrupt coming which calls get_cycles_sync() which uses the
> > 
> > Hmm, i didn't think NMI handlers called that. e.g. nmi watchdog just
> > uses jiffies.
> > 
> > get_cycles_sync patching happens only relatively early at boot, so oprofile
> > cannot be running yet.
> 
> Actually, the nmi handler does use the get_cycles(), and also uses the
>
> spinlock code:
> 
> arch/i386/kernel/nmi.c:
> __kprobes int nmi_watchdog_tick(struct pt_regs * regs, unsigned reason)
> ...
> static DEFINE_SPINLOCK(lock);   /* Serialise the printks */
> spin_lock(&lock);
> printk("NMI backtrace for cpu %d\n", cpu);
> ...
> spin_unlock(&lock);
> 
> If A - we change the spinlock code non atomically it would break.

It only has its lock prefixes twiggled, which should be ok.

>    B - printk reads the TSC to get a timestamp, it breaks:
>    it calls:
> printk_clock(void) -> sched_clock(); -> get_cycles_sync() on x86_64.

Are we reading the same source? sched_clock has never used get_cycles_sync(),
just ordinary get_cycles() which is not patched. In fact it mostly
used rdtscll() directly.

The main problem is alternative() nopify, e.g. for prefetches which
could hide in every list_for_each; but from a quick look the current
early NMI code doesn't do that.

> Yeah, that's a mess. That's why I always consider patching the code
> in a way that will let the NMI handler run through it in a sane manner
> _while_ the code is being patched. It implies _at least_ to do the
> updates atomically with atomic aligned memory writes that keeps the site
> being patched in a coherent state. Using a int3-based bypass is also
> required on Intel because of the erratum regarding instruction cache.

That's only for cross modifying code, no? 

> > This cannot happen for the current code: 
> > - full alternative patching happen only at boot when the other CPUs
> > are not running
> 
> Should be checked if NMIs and MCEs are active at that moment.

They are probably both.

I guess we could disable them again. I will cook up a patch.

> I see the mb()/rmb()/wmb() also uses alternatives, they should be
> checked for boot-time racing against NMIs and MCEs.

Patch above would take care of it.

> 
> init/main.c:start_kernel()
> 
> parse_args() (where the nmi watchdog is enabled it seems) would probably
> execute the smp-alt-boot and nmi_watchdog arguments in the order in which
> they are given as kernel arguments. So I guess it could race.

Not sure I see your point here. How can arguments race?

> 
> the "mce" kernel argument is also parsed in parse_args(), which leads to
> the same problem.

?


> 
> > For the immediate value patching it also cannot happen because
> > you'll never modify multiple instructions and all immediate values
> > can be changed atomically. 
> > 
> 
> Exactly, I always make sure that the immediate value within the
> instruction is aligned (so a 5 bytes movl must have an offset of +3
> compared to a 4 bytes alignment).

The x86 architecture doesn't require alignment for atomic updates.

> Make sure this API is used only to modify code meeting these
> requirements (those are the ones I remember from the top of my head):

Umm, that's far too complicated. Nobody will understand it anyways.
I'll cook up something simpler.

-Andi

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