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SubjectRe: [PATCH 3/3] early_printk: Add simple serialization to early_vprintk()
On Wed, 4 Oct 2017 11:08:30 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Oct 03, 2017 at 06:24:22PM -0400, Steven Rostedt wrote:
> > On Thu, 28 Sep 2017 14:18:26 +0200
> > Peter Zijlstra <peterz@infradead.org> wrote:
> > > static int early_vprintk(const char *fmt, va_list args)
> > > {
> > > + int n, cpu, old;
> > > char buf[512];
> > > +
> > > + cpu = get_cpu();
> > > + /*
> > > + * Test-and-Set inter-cpu spinlock with recursion.
> > > + */
> > > + for (;;) {
> > > + /*
> > > + * c-cas to avoid the exclusive bouncing on spin.
> > > + * Depends on the memory barrier implied by cmpxchg
> > > + * for ACQUIRE semantics.
> > > + */
> > > + old = READ_ONCE(early_printk_cpu);
> > > + if (old == -1) {
> >
> > If old != -1 and old != cpu, is it possible that the CPU could have
> > fetched an old value, and never try to fetch it again?
>
> What? If old != -1 and old != cpu, we'll hit the cpu_relax() and do the
> READ_ONCE() again. The READ_ONCE() guarantees we'll do the load again,
> as does the barrier() implied by cpu_relax().

I'm more worried about other architectures that don't have as strong of
a cache coherency.

[ Added Paul as he knows a lot about odd architectures ]

Is there any architecture that we support that can have the following:

CPU0 CPU1
---- ----
early_printk_cpu = 1
for (;;)
old = READ_ONCE(early_printk_cpu);
[ old = 1 ]

early_printk_cpu = -1

[...]
cpu_relax();
old = READ_ONCE(early_printk_cpu);

[ but the CPU uses the cache and not the memory? ]

old = 1;

-- Steve


>
> > The cmpxchg memory barrier only happens when old == -1.
>
> Yeah, so?
>
> > > + old = cmpxchg(&early_printk_cpu, -1, cpu);
> > > + if (old == -1)
> > > + break;
> > > + }
> > > + /*
> > > + * Allow recursion for interrupts and the like.
> > > + */
> > > + if (old == cpu)
> > > + break;
> > > +
> > > + cpu_relax();
> > > + }
> > >
> > > n = vscnprintf(buf, sizeof(buf), fmt, args);
> > > early_console->write(early_console, buf, n);
> > >
> > > + /*
> > > + * Unlock -- in case @old == @cpu, this is a no-op.
> > > + */
> > > + smp_store_release(&early_printk_cpu, old);
> > > + put_cpu();
> > > +
> > > return n;
> > > }

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