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SubjectRe: [PATCH 3/3] early_printk: Add simple serialization to early_vprintk()
On Thu, 28 Sep 2017 14:18:26 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> In order to avoid multiple CPUs banging on the serial port at the same
> time, add simple serialization. This explicitly deals with nested
> contexts (like IRQs etc.).
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> kernel/printk/printk.c | 35 ++++++++++++++++++++++++++++++++++-
> 1 file changed, 34 insertions(+), 1 deletion(-)
>
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -378,14 +378,47 @@ static int __init force_early_printk_set
> }
> early_param("force_early_printk", force_early_printk_setup);
>
> +static int early_printk_cpu = -1;
> +
> static int early_vprintk(const char *fmt, va_list args)
> {
> + int n, cpu, old;
> char buf[512];
> - int n;
> +
> + 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?

The cmpxchg memory barrier only happens when old == -1.

-- Steve

> + 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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