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    SubjectRe: [PATCH v5 3/9] x86/head: Move early exception panic code into early_fixup_exception
    On Mon, Apr 04, 2016 at 01:52:06PM +0200, Jan Kara wrote:
    > 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.

    There's a few printk() variants that do not go through this; which means
    they're broken for a number of cases, including the kdb printk
    redirection, this NMI stuff etc.

    > 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...

    You get lovely per char interleaving on you serial line ;-)

    What I've done in the past was something like the below; that way you
    only get the normal task->softirq->irq->nmi nesting, which is mostly
    decipherable.

    diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
    index bfbf284e4218..c4c3269ff104 100644
    --- a/kernel/printk/printk.c
    +++ b/kernel/printk/printk.c
    @@ -1907,17 +1907,36 @@ struct console *early_console;
    asmlinkage __visible void early_printk(const char *fmt, ...)
    {
    va_list ap;
    + static int print_cpu = -1;
    char buf[512];
    - int n;
    + int n, cpu;

    if (!early_console)
    return;

    + preempt_disable();
    + cpu = raw_smp_processor_id();
    + for (;;) {
    + if (READ_ONCE(print_cpu) == cpu)
    + break;
    +
    + if (READ_ONCE(print_cpu) == -1 &&
    + cmpxchg(&print_cpu, -1, cpu) == -1) {
    + cpu = -1;
    + break;
    + }
    +
    + cpu_relax();
    + }
    +
    va_start(ap, fmt);
    n = vscnprintf(buf, sizeof(buf), fmt, ap);
    va_end(ap);

    early_console->write(early_console, buf, n);
    +
    + smp_store_release(&print_cpu, cpu);
    + preempt_enable();
    }
    #endif

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