[PATCH] anti-cross-CPU printk/oops interleaving

[David Howells <dhowells@redhat.com>]

Hi Linus,

This patch prevents an oops on one CPU being interleaved char-by-char with
printks being performed on other CPUs, thus rendering them actually readable
under those circumstances.

David


diff -ur linux-2.5.74/kernel/printk.c linux-2.5.74-auto/kernel/printk.c
--- linux-2.5.74/kernel/printk.c	2003-07-03 15:37:24.000000000 +0100
+++ linux-2.5.74-auto/kernel/printk.c	2003-07-03 15:41:15.000000000 +0100
@@ -378,6 +378,9 @@
 		logged_chars++;
 }
 
+/* cpu currently holding logbuf_lock */
+static volatile int printk_cpu = -1;
+
 /*
  * This is printk.  It can be called from any context.  We want it to work.
  * 
@@ -400,8 +403,9 @@
 	static char printk_buf[1024];
 	static int log_level_unknown = 1;
 
-	if (oops_in_progress) {
-		/* If a crash is occurring, make sure we can't deadlock */
+	if (oops_in_progress && printk_cpu == smp_processor_id()) {
+		/* If a crash is occurring during printk() on this CPU,
+		 * make sure we can't deadlock */
 		spin_lock_init(&logbuf_lock);
 		/* And make sure that we print immediately */
 		init_MUTEX(&console_sem);
@@ -409,6 +413,7 @@
 
 	/* This stops the holder of console_sem just where we want him */
 	spin_lock_irqsave(&logbuf_lock, flags);
+	printk_cpu = smp_processor_id();
 
 	/* Emit the output into the temporary buffer */
 	va_start(args, fmt);