Re: [Xen-devel] [PATCH v5 3/9] x86/head: Move early exception panic code into early_fixup_exception

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

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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