Protect printf with spinlock to prevent interleaved output on SMP

On SMP systems, concurrent calls to printf() from multiple harts can
cause interleaved and unreadable output due to racing writes to the
shared output buffer.

Add a spinlock to serialize access to printf(), ensuring that only one
hart writes at a time.

This change improves the readability of debug messages and prevents
garbled output when multiple harts are active.

Author: visitorckw

lib/stdio.c

@@ -7,9 +7,13 @@
 
 #include <lib/libc.h>
 #include <stdarg.h>
+#include <spinlock.h>
 
 #include "private/stdio.h"
 
+static spinlock_t printf_lock = SPINLOCK_INITIALIZER;
+static uint32_t printf_flags = 0;
+
 /* Ignores output character, returns 0 (success). */
 static int stdout_null(int c)
 {
@@ -261,9 +265,11 @@ int32_t printf(const char *fmt, ...)
     va_list args;
     int32_t v;
 
+    spin_lock_irqsave(&printf_lock, &printf_flags);
     va_start(args, fmt);
     v = vsprintf(0, fmt, args);
     va_end(args);
+    spin_unlock_irqrestore(&printf_lock, printf_flags);
     return v;
 }