On Sat 02-04-16 13:58:19, Andy Lutomirski wrote:
> [cc Jan Kara]
>
> On Sat, Apr 2, 2016 at 1:47 PM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> > On Sat, Apr 02, 2016 at 01:13:37PM -0700, Andy Lutomirski wrote:
> >> Given that I this isn't really a regression with my patches (it
> >> probably never worked much better on 32-bit and the regs never would
> >> have shown at all on 64-bit),
> >
> > You're right. That thing calls printk *and* early_printk, WTF:
> >
> > #ifdef CONFIG_EARLY_PRINTK
> >
> > call early_printk
> > ...
> >
> > call dump_stack
> >
> > ...
> >
> > call __print_symbol
> >
> > those last two call printk. Great.
> >
> >> I propose a different approach: make
> >> printk work earlier. Something like:
> >>
> >> if (early) {
> >> early_printk(args);
> >> }
> >>
> >> or early_vprintk or whatever.
> >>
> >> If the cost of a branch mattered, this could be alternative-patched
> >> out later on, but that seems silly. I also bet that a more sensible
> >> fallback could be created in which printk would try to use an early
> >> console if there's no real console.
> >
> > So how about this:
> >
> > printk() does
> >
> > vprintk_func = this_cpu_read(printk_func);
> >
> > and that's
> >
> > DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default
> >
> > I guess we can make that function be early_printk-something and once
> > printk is initialized, we overwrite it with vprintk_default.
> >
> > Elegant and no need for if branches and alternatives.
> >
> > Hmmm.
>
> Jan, IIRC you were looking at printk recently-ish. Any thoughts here?
Sounds like a good idea to me. I've also consulted this with Petr Mladek
(added to CC) who is using printk_func per-cpu variable in his
printk-from-NMI patches and he also doesn't see a problem with this.
I was just wondering about one thing - this way we add more early printks
if I understand your intention right. Are we guaranteed that they happen
only from a single CPU? Because currently there is no locking in
early_printk() and thus we can end up writing to early console several
messages in parallel from different CPUs. Not sure what's going to happen
in that case...
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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