[PATCH] Provide better printk() support for SMP machines

The attached patch prevents oopses interleaving with characters from
other printks on other CPUs by only breaking the lock if the oops is
happening on the machine holding the lock.

It might be better if the oops generator got the lock and then called an
inner vprintk routine that assumed the caller holds the lock, thus
making oops reports "atomic".

Signed-Off-By: David Howells <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/printk.c

···
 	return r;
 }
 
+/* cpu currently holding logbuf_lock */
+static volatile unsigned int printk_cpu = UINT_MAX;
+
 asmlinkage int vprintk(const char *fmt, va_list args)
 {
 	unsigned long flags;
···
 	static char printk_buf[1024];
 	static int log_level_unknown = 1;
 
-	if (unlikely(oops_in_progress))
+	preempt_disable();
+	if (unlikely(oops_in_progress) && printk_cpu == smp_processor_id())
+		/* If a crash is occurring during printk() on this CPU,
+		 * make sure we can't deadlock */
 		zap_locks();
 
 	/* 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 */
 	printed_len = vscnprintf(printk_buf, sizeof(printk_buf), fmt, args);
···
 		 * CPU until it is officially up.  We shouldn't be calling into
 		 * random console drivers on a CPU which doesn't exist yet..
 		 */
+		printk_cpu = UINT_MAX;
 		spin_unlock_irqrestore(&logbuf_lock, flags);
 		goto out;
 	}
···
 		 * We own the drivers.  We can drop the spinlock and let
 		 * release_console_sem() print the text
 		 */
+		printk_cpu = UINT_MAX;
 		spin_unlock_irqrestore(&logbuf_lock, flags);
 		console_may_schedule = 0;
 		release_console_sem();
···
 		 * allows the semaphore holder to proceed and to call the
 		 * console drivers with the output which we just produced.
 		 */
+		printk_cpu = UINT_MAX;
 		spin_unlock_irqrestore(&logbuf_lock, flags);
 	}
 out:
+	preempt_enable();
 	return printed_len;
 }
 EXPORT_SYMBOL(printk);