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SubjectRe: [PATCH 3/3] early_printk: Add simple serialization to early_vprintk()
On Wed, Oct 04, 2017 at 09:04:01AM -0400, Steven Rostedt wrote:
> 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;

If you use READ_ONCE(), then all architectures I know of enforce
full ordering for accesses to a single variable. (If you don't use
READ_ONCE(), then in theory Itanium can reorder reads.) Me, I would
argue for WRITE_ONCE() as well to prevent store tearing.

It is only when you have at least two variables and at least two threads
than things start getting really "interesting". ;-)

Thanx, Paul

> -- 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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