printk: nbcon: Introduce KDB helpers

These helpers will be used when calling console->write_atomic on
KDB code in the next patch. It's basically the same implementation
as nbcon_device_try_acquire, but using NBCON_PRIO_EMERGENCY when
acquiring the context.

If the acquire succeeds, the message and message length are assigned to
nbcon_write_context so ->write_atomic can print the message.

After release try to flush the console since there may be a backlog of
messages in the ringbuffer. The kthread console printers do not get a
chance to run while kdb is active.

Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Link: https://patch.msgid.link/20251016-nbcon-kgdboc-v6-2-866aac60a80e@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>

diff --git a/include/linux/console.h b/include/linux/console.h
index 5c3a718c22fcc4cf28a557a4c13ed2f9123c0e5c..9406342b27db463a4d0445a539c80a2729c5cb6b 100644 (file)
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -606,6 +606,9 @@ extern bool nbcon_can_proceed(struct nbcon_write_context *wctxt);
 extern bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt);
 extern bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt);
 extern void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt);
+extern bool nbcon_kdb_try_acquire(struct console *con,
+                                 struct nbcon_write_context *wctxt);
+extern void nbcon_kdb_release(struct nbcon_write_context *wctxt);
 
 /*
  * Check if the given console is currently capable and allowed to print
@@ -655,6 +658,9 @@ static inline bool nbcon_can_proceed(struct nbcon_write_context *wctxt) { return
 static inline bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt) { return false; }
 static inline bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt) { return false; }
 static inline void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt) { }
+static inline bool nbcon_kdb_try_acquire(struct console *con,
+                                        struct nbcon_write_context *wctxt) { return false; }
+static inline void nbcon_kdb_release(struct console *con) { }
 static inline bool console_is_usable(struct console *con, short flags,
                                     bool use_atomic) { return false; }
 #endif

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 558ef31779760340ce42608294d91d5401239f1d..e1bf5409cb6bd39c5af6774569efa79bffe262df 100644 (file)
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1855,3 +1855,64 @@ void nbcon_device_release(struct console *con)
        console_srcu_read_unlock(cookie);
 }
 EXPORT_SYMBOL_GPL(nbcon_device_release);
+
+/**
+ * nbcon_kdb_try_acquire - Try to acquire nbcon console and enter unsafe
+ *                        section
+ * @con:       The nbcon console to acquire
+ * @wctxt:     The nbcon write context to be used on success
+ *
+ * Context:    Under console_srcu_read_lock() for emitting a single kdb message
+ *             using the given con->write_atomic() callback. Can be called
+ *             only when the console is usable at the moment.
+ *
+ * Return:     True if the console was acquired. False otherwise.
+ *
+ * kdb emits messages on consoles registered for printk() without
+ * storing them into the ring buffer. It has to acquire the console
+ * ownerhip so that it could call con->write_atomic() callback a safe way.
+ *
+ * This function acquires the nbcon console using priority NBCON_PRIO_EMERGENCY
+ * and marks it unsafe for handover/takeover.
+ */
+bool nbcon_kdb_try_acquire(struct console *con,
+                          struct nbcon_write_context *wctxt)
+{
+       struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+       memset(ctxt, 0, sizeof(*ctxt));
+       ctxt->console = con;
+       ctxt->prio    = NBCON_PRIO_EMERGENCY;
+
+       if (!nbcon_context_try_acquire(ctxt, false))
+               return false;
+
+       if (!nbcon_context_enter_unsafe(ctxt))
+               return false;
+
+       return true;
+}
+
+/**
+ * nbcon_kdb_release - Exit unsafe section and release the nbcon console
+ *
+ * @wctxt:     The nbcon write context initialized by a successful
+ *             nbcon_kdb_try_acquire()
+ */
+void nbcon_kdb_release(struct nbcon_write_context *wctxt)
+{
+       struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+       if (!nbcon_context_exit_unsafe(ctxt))
+               return;
+
+       nbcon_context_release(ctxt);
+
+       /*
+        * Flush any new printk() messages added when the console was blocked.
+        * Only the console used by the given write context was blocked.
+        * The console was locked only when the write_atomic() callback
+        * was usable.
+        */
+       __nbcon_atomic_flush_pending_con(ctxt->console, prb_next_reserve_seq(prb), false);
+}