Merge tag 'printk-for-5.19' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

 - Offload writing printk() messages on consoles to per-console
   kthreads.

   It prevents soft-lockups when an extensive amount of messages is
   printed. It was observed, for example, during boot of large systems
   with a lot of peripherals like disks or network interfaces.

   It prevents live-lockups that were observed, for example, when
   messages about allocation failures were reported and a CPU handled
   consoles instead of reclaiming the memory. It was hard to solve even
   with rate limiting because it would need to take into account the
   amount of messages and the speed of all consoles.

   It is a must to have for real time. Otherwise, any printk() might
   break latency guarantees.

   The per-console kthreads allow to handle each console on its own
   speed. Slow consoles do not longer slow down faster ones. And
   printk() does not longer unpredictably slows down various code paths.

   There are situations when the kthreads are either not available or
   not reliable, for example, early boot, suspend, or panic. In these
   situations, printk() uses the legacy mode and tries to handle
   consoles immediately.

 - Add documentation for the printk index.

* tag 'printk-for-5.19' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
  printk, tracing: fix console tracepoint
  printk: remove @console_locked
  printk: extend console_lock for per-console locking
  printk: add kthread console printers
  printk: add functions to prefer direct printing
  printk: add pr_flush()
  printk: move buffer definitions into console_emit_next_record() caller
  printk: refactor and rework printing logic
  printk: add con_printk() macro for console details
  printk: call boot_delay_msec() in printk_delay()
  printk: get caller_id/timestamp after migration disable
  printk: wake waiters for safe and NMI contexts
  printk: wake up all waiters
  printk: add missing memory barrier to wake_up_klogd()
  printk: cpu sync always disable interrupts
  printk: rename cpulock functions
  printk/index: Printk index feature documentation
  MAINTAINERS: Add printk indexing maintainers on mention of printk_index

1 files changed, 19 insertions, 0 deletions

diff --git a/include/linux/console.h b/include/linux/console.h
index 7cd758a4f44e..143653090c48 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -16,6 +16,7 @@
 
 #include <linux/atomic.h>
 #include <linux/types.h>
+#include <linux/mutex.h>
 
 struct vc_data;
 struct console_font_op;
@@ -151,6 +152,24 @@ struct console {
 	int	cflag;
 	uint	ispeed;
 	uint	ospeed;
+	u64	seq;
+	unsigned long dropped;
+	struct task_struct *thread;
+	bool	blocked;
+
+	/*
+	 * The per-console lock is used by printing kthreads to synchronize
+	 * this console with callers of console_lock(). This is necessary in
+	 * order to allow printing kthreads to run in parallel to each other,
+	 * while each safely accessing the @blocked field and synchronizing
+	 * against direct printing via console_lock/console_unlock.
+	 *
+	 * Note: For synchronizing against direct printing via
+	 *       console_trylock/console_unlock, see the static global
+	 *       variable @console_kthreads_active.
+	 */
+	struct mutex lock;
+
 	void	*data;
 	struct	 console *next;
 };