Provide better printk() support for SMP machines

diff -uNrp linux-2.6.12-mm1/kernel/printk.c linux-2.6.12-mm1-cachefs-wander/kernel/printk.c
@@ -514,6 +514,9 @@ asmlinkage int printk(const char *fmt, .
 	return r;
 }
 
+/* cpu currently holding logbuf_lock */
+static volatile int printk_cpu = -1;
+
 asmlinkage int vprintk(const char *fmt, va_list args)
 {
 	unsigned long flags;

@@ -522,11 +525,15 @@ asmlinkage int vprintk(const char *fmt, 
 	static char printk_buf[1024];
 	static int log_level_unknown = 1;
 
-	if (unlikely(oops_in_progress))
+	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();
+	smp_wmb();
 
 	/* Emit the output into the temporary buffer */
 	printed_len = vscnprintf(printk_buf, sizeof(printk_buf), fmt, args);

@@ -595,6 +602,7 @@ asmlinkage int vprintk(const char *fmt, 
 		 * 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 = -1;
 		spin_unlock_irqrestore(&logbuf_lock, flags);
 		goto out;
 	}

@@ -604,6 +612,7 @@ asmlinkage int vprintk(const char *fmt, 
 		 * We own the drivers.  We can drop the spinlock and let
 		 * release_console_sem() print the text
 		 */
+		printk_cpu = -1;
 		spin_unlock_irqrestore(&logbuf_lock, flags);
 		console_may_schedule = 0;
 		release_console_sem();

@@ -613,6 +622,7 @@ asmlinkage int vprintk(const char *fmt, 
 		 * allows the semaphore holder to proceed and to call the
 		 * console drivers with the output which we just produced.
 		 */
+		printk_cpu = -1;
 		spin_unlock_irqrestore(&logbuf_lock, flags);
 	}
 out:

diff -uNrp linux-2.6.13-rc2-mm1/kernel/printk.c linux-2.6.13-rc2-mm1-cachefs/kernel/printk.c
@@ -514,6 +514,9 @@ asmlinkage int printk(const char *fmt, .
 	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;

@@ -522,11 +525,15 @@ asmlinkage int vprintk(const char *fmt, 
 	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 == raw_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 = raw_smp_processor_id();
 
 	/* Emit the output into the temporary buffer */
 	printed_len = vscnprintf(printk_buf, sizeof(printk_buf), fmt, args);

@@ -588,13 +595,14 @@ asmlinkage int vprintk(const char *fmt, 
 			log_level_unknown = 1;
 	}
 
-	if (!cpu_online(smp_processor_id())) {
+	if (!cpu_online(raw_smp_processor_id())) {
 		/*
 		 * Some console drivers may assume that per-cpu resources have
 		 * been allocated.  So don't allow them to be called by this
 		 * 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;
 	}

@@ -604,6 +612,7 @@ asmlinkage int vprintk(const char *fmt, 
 		 * 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();

@@ -613,9 +622,11 @@ asmlinkage int vprintk(const char *fmt, 
 		 * 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);