Merge tag 'printk-for-6.14' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

 - Prevent possible deadlocks, caused by the lock serializing per-CPU
   backtraces, by entering the deferred printk context

 - Enforce the right casting in LOG_BUF_LEN_MAX definition

* tag 'printk-for-6.14' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
  printk: Defer legacy printing when holding printk_cpu_sync
  printk: Remove redundant deferred check in vprintk()
  printk: Fix signed integer overflow when defining LOG_BUF_LEN_MAX

Author: torvalds

kernel/printk/internal.h

@@ -338,3 +338,9 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped);
 void console_prepend_replay(struct printk_message *pmsg);
 #endif
+
+#ifdef CONFIG_SMP
+bool is_printk_cpu_sync_owner(void);
+#else
+static inline bool is_printk_cpu_sync_owner(void) { return false; }
+#endif

kernel/printk/printk.c

@@ -523,7 +523,7 @@ static struct latched_seq clear_seq = {
 /* record buffer */
 #define LOG_ALIGN __alignof__(unsigned long)
 #define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
-#define LOG_BUF_LEN_MAX (u32)(1 << 31)
+#define LOG_BUF_LEN_MAX ((u32)1 << 31)
 static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
 static char *log_buf = __log_buf;
 static u32 log_buf_len = __LOG_BUF_LEN;
@@ -4922,6 +4922,11 @@ void console_try_replay_all(void)
 static atomic_t printk_cpu_sync_owner = ATOMIC_INIT(-1);
 static atomic_t printk_cpu_sync_nested = ATOMIC_INIT(0);
 
+bool is_printk_cpu_sync_owner(void)
+{
+	return (atomic_read(&printk_cpu_sync_owner) == raw_smp_processor_id());
+}
+
 /**
  * __printk_cpu_sync_wait() - Busy wait until the printk cpu-reentrant
  *                            spinning lock is not owned by any CPU.

kernel/printk/printk_safe.c

@@ -61,10 +61,15 @@ bool is_printk_legacy_deferred(void)
 	/*
 	 * The per-CPU variable @printk_context can be read safely in any
 	 * context. CPU migration is always disabled when set.
+	 *
+	 * A context holding the printk_cpu_sync must not spin waiting for
+	 * another CPU. For legacy printing, it could be the console_lock
+	 * or the port lock.
 	 */
 	return (force_legacy_kthread() ||
 		this_cpu_read(printk_context) ||
-		in_nmi());
+		in_nmi() ||
+		is_printk_cpu_sync_owner());
 }
 
 asmlinkage int vprintk(const char *fmt, va_list args)
@@ -74,15 +79,6 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0))
 		return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
 #endif
-
-	/*
-	 * Use the main logbuf even in NMI. But avoid calling console
-	 * drivers that might have their own locks.
-	 */
-	if (is_printk_legacy_deferred())
-		return vprintk_deferred(fmt, args);
-
-	/* No obstacles. */
 	return vprintk_default(fmt, args);
 }
 EXPORT_SYMBOL(vprintk);