[PATCH 3/3] early_printk: Add simple serialization to early_vprintk()

[Peter Zijlstra]

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) <pet...@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) {
+                       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;
 }