Hi,
After more detailed runtime testing I discovered that I didn't re-insert
the console to the correct place in the list. More below...
On 2022-11-14, John Ogness [off-list ref] wrote:
quoted hunk ↗ jump to hunk
diff --git a/include/linux/console.h b/include/linux/console.h
index f716e1dd9eaf..9cea254b34b8 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -291,6 +291,7 @@ enum con_flush_mode {
};
extern int add_preferred_console(char *name, int idx, char *options);
+extern void console_force_preferred_locked(struct console *con);
extern void register_console(struct console *);
extern int unregister_console(struct console *);
extern void console_lock(void);diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e770b1ede6c9..dff76c1cef80 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3461,6 +3462,48 @@ int unregister_console(struct console *console)
}
EXPORT_SYMBOL(unregister_console);
+/**
+ * console_force_preferred_locked - force a registered console preferred
+ * @con: The registered console to force preferred.
+ *
+ * Must be called under console_list_lock().
+ */
+void console_force_preferred_locked(struct console *con)
+{
+ struct console *cur_pref_con;
+
+ if (!console_is_registered_locked(con))
+ return;
+
+ cur_pref_con = console_first();
+
+ /* Already preferred? */
+ if (cur_pref_con == con)
+ return;
+
+ /*
+ * Delete, but do not re-initialize the entry. This allows the console
+ * to continue to appear registered (via any hlist_unhashed_lockless()
+ * checks), even though it was briefly removed from the console list.
+ */
+ hlist_del_rcu(&con->node);
+
+ /*
+ * Ensure that all SRCU list walks have completed so that the console
+ * can be added to the beginning of the console list and its forward
+ * list pointer can be re-initialized.
+ */
+ synchronize_srcu(&console_srcu);
+
+ con->flags |= CON_CONSDEV;
+ WARN_ON(!con->device);
+
+ /* Only the new head can have CON_CONSDEV set. */
+ console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV);
+ hlist_add_behind_rcu(&con->node, console_list.first);
This is adding the console as the 2nd item. It should be the new
head. The patch below fixes it.
I have done careful runtime testing with this fixup. After the
force_preferred, the console is the new head and sending data to
/dev/console redirects to that console.
It would be nice if we could fold this in. Sorry.
John Ogness
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8d635467882f..4b77586cf4cb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3494,7 +3494,7 @@ void console_force_preferred_locked(struct console *con)
/* Only the new head can have CON_CONSDEV set. */
console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV);
- hlist_add_behind_rcu(&con->node, console_list.first);
+ hlist_add_head_rcu(&con->node, &console_list);
}
EXPORT_SYMBOL(console_force_preferred_locked);