Re: [patch 1/8] Immediate Values - Global Modules List and Module Mutex

[Rusty Russell <rusty@rustcorp.com.au>]

On Tue, 2007-09-11 at 10:27 -0400, Mathieu Desnoyers wrote: 
> * Rusty Russell (rusty@rustcorp.com.au) wrote:
> > On Mon, 2007-09-10 at 20:45 -0400, Mathieu Desnoyers wrote:
> > > Code patching of _live_ SMP code is allowed. This is why I went through
> > > all this trouble on i386.
> > 
> > Oh, I was pretty sure it wasn't.  OK.
> > 
> > So now why three versions of immediate_set()?  And why are you using my
> > lock for exclusion?  Against what?
> > 
> 
> If we need to patch code at boot time, when interrupts are still
> disabled (it happens when we parse the kernel arguments for instance),
> we cannot afford to use IPIs to call sync_core() on each cpu, using
> breakpoints/notifier chains could be tricky (because we are very early
> at boot and alternatives or paravirt may not have been applied yet).

Hi Mathieu,

Sure, but why is that the caller's problem?  immediate_set() isn't
fastpath, so why not make it do an "if (early_boot)" internally?

> _immediate_set() has been introduced because of the way immediate values
> are used by markers: the linux kernel markers already hold the module
> mutex when they need to update the immediate values. Taking the mutex
> twice makes no sence, so _immediate_set() is used when the caller
> already holds the module mutex.

> Why not just have one immediate_set() which iterates through and fixes
> > up all the references?
> 
> (reasons explained above)
> 
> > It can use an internal lock if you want to avoid
> > concurrent immediate_set() calls.
> > 
> 
> An internal lock won't protect against modules load/unload race. We have
> to iterate on the module list.

Sure, but it seems like that's fairly easy to do within module.c:

/* This updates all the immediates even though only one might have
* changed.  But it's so rare it's not worth optimizing. */
void module_update_immediates(void)
{
mutex_lock(&module_mutex);
list_for_each_entry(mod, &modules, list)
update_immediates(mod->immediate, mod->num_immediate);
mutex_unlock(&module_mutex);
}

Then during module load you do:

update_immediates(mod->immediate, mod->num_immediate);

Your immediate_update() just becomes:

update_immediates(__start___immediate,
		  __stop___immediate - __start___immediate);
module_update_immediates();

update_immediates() can grab the immediate_mutex if you want.

> > Why is it easier to patch the sites now than later?  Currently it's just
> > churn.  You could go back and find them when this mythical patch gets
> > merged into this mythical future gcc version.  It could well need a
> > completely different macro style, like "cond_imm(var, code)".
> 
> Maybe you're right. My though was that if we have a way to express a
> strictly boolean if() statement that can later be optimized further by
> gcc using a jump rather than a conditionnal branch and currently emulate
> it by using a load immediate/test/branch, we might want to do so right
> now so we don't have to do a second code transition from
> if (immediate_read(&var)) to immediate_if (&var) later. But you might be
> right in that the form could potentially change anyway when the
> implementation would come, although I don't see how.

I was thinking that we might find useful specific cases before we get
GCC support, which archs can override with tricky asm if they wish.

Cheers,
Rusty.