Merge tag 'printk-for-6.9' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:
 "Improve the behavior during panic. The issues were found when testing
  the ongoing changes introducing atomic consoles and printk kthreads:

   - pr_flush() has to wait for the last reserved record instead of the
     last finalized one. Note that records are finalized in random order
     when generated by more CPUs in parallel.

   - Ignore non-finalized records during panic(). Messages printed on
     panic-CPU are always finalized. Messages printed by other CPUs
     might never be finalized when the CPUs get stopped.

   - Block new printk() calls on non-panic CPUs completely. Backtraces
     are printed before entering the panic mode. Later messages would
     just mess information printed by the panic CPU.

   - Do not take console_lock in console_flush_on_panic() at all. The
     original code did try_lock()/console_unlock(). The unlock part
     might cause a deadlock when panic() happened in a scheduler code.

   - Fix conversion of 64-bit sequence number for 32-bit atomic
     operations"

* tag 'printk-for-6.9' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
  dump_stack: Do not get cpu_sync for panic CPU
  panic: Flush kernel log buffer at the end
  printk: Avoid non-panic CPUs writing to ringbuffer
  printk: Disable passing console lock owner completely during panic()
  printk: ringbuffer: Skip non-finalized records in panic
  printk: Wait for all reserved records with pr_flush()
  printk: ringbuffer: Cleanup reader terminology
  printk: Add this_cpu_in_panic()
  printk: For @suppress_panic_printk check for other CPU in panic
  printk: ringbuffer: Clarify special lpos values
  printk: ringbuffer: Do not skip non-finalized records with prb_next_seq()
  printk: Use prb_first_seq() as base for 32bit seq macros
  printk: Adjust mapping for 32bit seq macros
  printk: nbcon: Relocate 32bit seq macros

1 files changed, 59 insertions, 42 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1c6e7dfc4ba7..b06f63e276c1 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -347,6 +347,29 @@ static bool panic_in_progress(void)
 	return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
 }
 
+/* Return true if a panic is in progress on the current CPU. */
+bool this_cpu_in_panic(void)
+{
+	/*
+	 * We can use raw_smp_processor_id() here because it is impossible for
+	 * the task to be migrated to the panic_cpu, or away from it. If
+	 * panic_cpu has already been set, and we're not currently executing on
+	 * that CPU, then we never will be.
+	 */
+	return unlikely(atomic_read(&panic_cpu) == raw_smp_processor_id());
+}
+
+/*
+ * Return true if a panic is in progress on a remote CPU.
+ *
+ * On true, the local CPU should immediately release any printing resources
+ * that may be needed by the panic CPU.
+ */
+bool other_cpu_in_panic(void)
+{
+	return (panic_in_progress() && !this_cpu_in_panic());
+}
+
 /*
  * This is used for debugging the mess that is the VT code by
  * keeping track if we have the console semaphore held. It's
@@ -439,12 +462,6 @@ static int console_msg_format = MSG_FORMAT_DEFAULT;
 static DEFINE_MUTEX(syslog_lock);
 
 #ifdef CONFIG_PRINTK
-/*
- * During panic, heavy printk by other CPUs can delay the
- * panic and risk deadlock on console resources.
- */
-static int __read_mostly suppress_panic_printk;
-
 DECLARE_WAIT_QUEUE_HEAD(log_wait);
 /* All 3 protected by @syslog_lock. */
 /* the next printk record to read by syslog(READ) or /proc/kmsg */
@@ -1835,10 +1852,23 @@ static bool console_waiter;
  */
 static void console_lock_spinning_enable(void)
 {
+	/*
+	 * Do not use spinning in panic(). The panic CPU wants to keep the lock.
+	 * Non-panic CPUs abandon the flush anyway.
+	 *
+	 * Just keep the lockdep annotation. The panic-CPU should avoid
+	 * taking console_owner_lock because it might cause a deadlock.
+	 * This looks like the easiest way how to prevent false lockdep
+	 * reports without handling races a lockless way.
+	 */
+	if (panic_in_progress())
+		goto lockdep;
+
 	raw_spin_lock(&console_owner_lock);
 	console_owner = current;
 	raw_spin_unlock(&console_owner_lock);
 
+lockdep:
 	/* The waiter may spin on us after setting console_owner */
 	spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
 }
@@ -1863,6 +1893,22 @@ static int console_lock_spinning_disable_and_check(int cookie)
 {
 	int waiter;
 
+	/*
+	 * Ignore spinning waiters during panic() because they might get stopped
+	 * or blocked at any time,
+	 *
+	 * It is safe because nobody is allowed to start spinning during panic
+	 * in the first place. If there has been a waiter then non panic CPUs
+	 * might stay spinning. They would get stopped anyway. The panic context
+	 * will never start spinning and an interrupted spin on panic CPU will
+	 * never continue.
+	 */
+	if (panic_in_progress()) {
+		/* Keep lockdep happy. */
+		spin_release(&console_owner_dep_map, _THIS_IP_);
+		return 0;
+	}
+
 	raw_spin_lock(&console_owner_lock);
 	waiter = READ_ONCE(console_waiter);
 	console_owner = NULL;
@@ -2259,8 +2305,12 @@ asmlinkage int vprintk_emit(int facility, int level,
 	if (unlikely(suppress_printk))
 		return 0;
 
-	if (unlikely(suppress_panic_printk) &&
-	    atomic_read(&panic_cpu) != raw_smp_processor_id())
+	/*
+	 * The messages on the panic CPU are the most important. If
+	 * non-panic CPUs are generating any messages, they will be
+	 * silently dropped.
+	 */
+	if (other_cpu_in_panic())
 		return 0;
 
 	if (level == LOGLEVEL_SCHED) {
@@ -2590,26 +2640,6 @@ static int console_cpu_notify(unsigned int cpu)
 	return 0;
 }
 
-/*
- * Return true if a panic is in progress on a remote CPU.
- *
- * On true, the local CPU should immediately release any printing resources
- * that may be needed by the panic CPU.
- */
-bool other_cpu_in_panic(void)
-{
-	if (!panic_in_progress())
-		return false;
-
-	/*
-	 * We can use raw_smp_processor_id() here because it is impossible for
-	 * the task to be migrated to the panic_cpu, or away from it. If
-	 * panic_cpu has already been set, and we're not currently executing on
-	 * that CPU, then we never will be.
-	 */
-	return atomic_read(&panic_cpu) != raw_smp_processor_id();
-}
-
 /**
  * console_lock - block the console subsystem from printing
  *
@@ -2765,8 +2795,6 @@ void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped)
 bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 			     bool is_extended, bool may_suppress)
 {
-	static int panic_console_dropped;
-
 	struct printk_buffers *pbufs = pmsg->pbufs;
 	const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf);
 	const size_t outbuf_sz = sizeof(pbufs->outbuf);
@@ -2794,17 +2822,6 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 	pmsg->seq = r.info->seq;
 	pmsg->dropped = r.info->seq - seq;
 
-	/*
-	 * Check for dropped messages in panic here so that printk
-	 * suppression can occur as early as possible if necessary.
-	 */
-	if (pmsg->dropped &&
-	    panic_in_progress() &&
-	    panic_console_dropped++ > 10) {
-		suppress_panic_printk = 1;
-		pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n");
-	}
-
 	/* Skip record that has level above the console loglevel. */
 	if (may_suppress && suppress_message_printing(r.info->level))
 		goto out;
@@ -3750,7 +3767,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 
 	might_sleep();
 
-	seq = prb_next_seq(prb);
+	seq = prb_next_reserve_seq(prb);
 
 	/* Flush the consoles so that records up to @seq are printed. */
 	console_lock();