Merge tag 'printk-for-6.2' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

 - Add NMI-safe SRCU reader API. It uses atomic_inc() instead of
   this_cpu_inc() on strong load-store architectures.

 - Introduce new console_list_lock to synchronize a manipulation of the
   list of registered consoles and their flags.

   This is a first step in removing the big-kernel-lock-like behavior of
   console_lock(). This semaphore still serializes console->write()
   calbacks against:

      - each other. It primary prevents potential races between early
        and proper console drivers using the same device.

      - suspend()/resume() callbacks and init() operations in some
        drivers.

      - various other operations in the tty/vt and framebufer
        susbsystems. It is likely that console_lock() serializes even
        operations that are not directly conflicting with the
        console->write() callbacks here. This is the most complicated
        big-kernel-lock aspect of the console_lock() that will be hard
        to untangle.

 - Introduce new console_srcu lock that is used to safely iterate and
   access the registered console drivers under SRCU read lock.

   This is a prerequisite for introducing atomic console drivers and
   console kthreads. It will reduce the complexity of serialization
   against normal consoles and console_lock(). Also it should remove the
   risk of deadlock during critical situations, like Oops or panic, when
   only atomic consoles are registered.

 - Check whether the console is registered instead of enabled on many
   locations. It was a historical leftover.

 - Cleanly force a preferred console in xenfb code instead of a dirty
   hack.

 - A lot of code and comment clean ups and improvements.

* tag 'printk-for-6.2' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux: (47 commits)
  printk: htmldocs: add missing description
  tty: serial: sh-sci: use setup() callback for early console
  printk: relieve console_lock of list synchronization duties
  tty: serial: kgdboc: use console_list_lock to trap exit
  tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()
  tty: serial: kgdboc: use console_list_lock for list traversal
  tty: serial: kgdboc: use srcu console list iterator
  proc: consoles: use console_list_lock for list iteration
  tty: tty_io: use console_list_lock for list synchronization
  printk, xen: fbfront: create/use safe function for forcing preferred
  netconsole: avoid CON_ENABLED misuse to track registration
  usb: early: xhci-dbc: use console_is_registered()
  tty: serial: xilinx_uartps: use console_is_registered()
  tty: serial: samsung_tty: use console_is_registered()
  tty: serial: pic32_uart: use console_is_registered()
  tty: serial: earlycon: use console_is_registered()
  tty: hvc: use console_is_registered()
  efi: earlycon: use console_is_registered()
  tty: nfcon: use console_is_registered()
  serial_core: replace uart_console_enabled() with uart_console_registered()
  ...

1 files changed, 16 insertions, 2 deletions

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 67d3c48a1522..5c7e9ba7cd6b 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -545,6 +545,7 @@ static void kdb_msg_write(const char *msg, int msg_len)
 {
 	struct console *c;
 	const char *cp;
+	int cookie;
 	int len;
 
 	if (msg_len == 0)
@@ -558,8 +559,20 @@ static void kdb_msg_write(const char *msg, int msg_len)
 		cp++;
 	}
 
-	for_each_console(c) {
-		if (!(c->flags & CON_ENABLED))
+	/*
+	 * The console_srcu_read_lock() only provides safe console list
+	 * traversal. The use of the ->write() callback relies on all other
+	 * CPUs being stopped at the moment and console drivers being able to
+	 * handle reentrance when @oops_in_progress is set.
+	 *
+	 * There is no guarantee that every console driver can handle
+	 * reentrance in this way; the developer deploying the debugger
+	 * is responsible for ensuring that the console drivers they
+	 * have selected handle reentrance appropriately.
+	 */
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(c) {
+		if (!(console_srcu_read_flags(c) & CON_ENABLED))
 			continue;
 		if (c == dbg_io_ops->cons)
 			continue;
@@ -577,6 +590,7 @@ static void kdb_msg_write(const char *msg, int msg_len)
 		--oops_in_progress;
 		touch_nmi_watchdog();
 	}
+	console_srcu_read_unlock(cookie);
 }
 
 int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)