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