[PATCH] printk: nbcon: Prevent deadlock when flushing nbcon consoles in the legacy loop

[Petr Mladek]

Ignore pending records in nbcon consoles in the legacy console_flush_all()
loop when the nbcon console context can't be acquired. They will be
flushed either by the current nbcon context owner or to-be-introduced
printk kthread.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 kernel/printk/nbcon.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index c57ad34a8d56..88c40f165be4 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -964,8 +964,12 @@ static __ref unsigned int *nbcon_get_cpu_emergency_nesting(void)
  *				write_atomic() callback
  * @wctxt:	An initialized write context struct to use for this context
  *
- * Return:	False if it is known there are no more records to print,
- *		otherwise true.
+ * Return:	True, when a record has been printed and there are still
+ *		pending records. The caller might want to continue flushing.
+ *
+ *		False, when there is no pending record, or when the console
+ *		context can't be acquired, or the ownership has been lost.
+ *		The caller should give up. Either the job is or	can't be done.
  *
  * This is an internal helper to handle the locking of the console before
  * calling nbcon_emit_next_record().
@@ -975,7 +979,7 @@ static bool nbcon_atomic_emit_one(struct nbcon_write_context *wctxt)
 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
 
 	if (!nbcon_context_try_acquire(ctxt))
-		return true;
+		return false;
 
 	/*
 	 * nbcon_emit_next_record() returns false when the console was
@@ -983,7 +987,7 @@ static bool nbcon_atomic_emit_one(struct nbcon_write_context *wctxt)
 	 * longer valid.
 	 */
 	if (!nbcon_emit_next_record(wctxt))
-		return true;
+		return false;
 
 	nbcon_context_release(ctxt);
 
@@ -1023,8 +1027,13 @@ enum nbcon_prio nbcon_get_default_prio(void)
  * @cookie:	The cookie from the SRCU read lock.
  *
  * Context:	Any context except NMI.
- * Return:	False if the given console has no next record to print,
- *		otherwise true.
+ *
+ * Return:	True, when a record has been printed and there are still
+ *		pending records. The caller might want to continue flushing.
+ *
+ *		False, when there is no pending record, or when the console
+ *		context can't be acquired, or the ownership has been lost.
+ *		The caller should give up. Either the job is or	can't be done.
  *
  * This function is meant to be called by console_flush_all() to print records
  * on nbcon consoles from legacy context (printing via console unlocking).
-- 
2.44.0


Best Regards,
Petr