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, 13 insertions, 3 deletions

diff --git a/lib/dump_stack.c b/lib/dump_stack.c
index 83471e81501a..222c6d6c8281 100644
--- a/lib/dump_stack.c
+++ b/lib/dump_stack.c
@@ -96,15 +96,25 @@ static void __dump_stack(const char *log_lvl)
  */
 asmlinkage __visible void dump_stack_lvl(const char *log_lvl)
 {
+	bool in_panic = this_cpu_in_panic();
 	unsigned long flags;
 
 	/*
 	 * Permit this cpu to perform nested stack dumps while serialising
-	 * against other CPUs
+	 * against other CPUs, unless this CPU is in panic.
+	 *
+	 * When in panic, non-panic CPUs are not permitted to store new
+	 * printk messages so there is no need to synchronize the output.
+	 * This avoids potential deadlock in panic() if another CPU is
+	 * holding and unable to release the printk_cpu_sync.
 	 */
-	printk_cpu_sync_get_irqsave(flags);
+	if (!in_panic)
+		printk_cpu_sync_get_irqsave(flags);
+
 	__dump_stack(log_lvl);
-	printk_cpu_sync_put_irqrestore(flags);
+
+	if (!in_panic)
+		printk_cpu_sync_put_irqrestore(flags);
 }
 EXPORT_SYMBOL(dump_stack_lvl);