Subject: [PATCH 10/24] printk: nobkl: Add emit function and callback functions for atomic printing
From: Thomas Gleixner <[email-masked]>
Date: Sun, 11 Sep 2022 00:28:10 +0200

Implement an emit function for non-BKL consoles to output printk
messages. It utilizes the lockless printk_get_next_message() and
console_prepend_dropped() functions to retrieve/build the output
message. The emit function includes the required safety points to
check for handover/takeover and calls a new write_atomic callback
of the console driver to output the message. It also includes proper
handling for updating the non-BKL console sequence number.

Co-developed-by: John Ogness <[email-masked]>
Signed-off-by: John Ogness <[email-masked]>
Signed-off-by: Thomas Gleixner (Intel) <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 include/linux/console.h      |    8 ++
 kernel/printk/internal.h     |   10 +++
 kernel/printk/printk.c       |   12 +---
 kernel/printk/printk_nobkl.c |  121 ++++++++++++++++++++++++++++++++++++++++++-
 4 files changed, 142 insertions(+), 9 deletions(-)

Index: linux-6.3.0-rt11/include/linux/console.h
===================================================================
--- linux-6.3.0-rt11.orig/include/linux/console.h
+++ linux-6.3.0-rt11/include/linux/console.h
@@ -251,10 +251,12 @@ struct printk_buffers;
  * @newseq:		The sequence number for progress
  * @prio:		Priority of the context
  * @pbufs:		Pointer to the text buffer for this context
+ * @dropped:		Dropped counter for the current context
  * @thread:		The acquire is printk thread context
  * @hostile:		Hostile takeover requested. Cleared on normal
  *			acquire or friendly handover
  * @spinwait:		Spinwait on acquire if possible
+ * @backlog:		Ringbuffer has pending records
  */
 struct cons_context {
 	struct console		*console;
@@ -267,9 +269,11 @@ struct cons_context {
 	unsigned int		spinwait_max_us;
 	enum cons_prio		prio;
 	struct printk_buffers	*pbufs;
+	unsigned long		dropped;
 	unsigned int		thread		: 1;
 	unsigned int		hostile		: 1;
 	unsigned int		spinwait	: 1;
+	unsigned int		backlog		: 1;
 };
 
 /**
@@ -311,6 +315,7 @@ struct cons_context_data;
  * @atomic_state:	State array for NOBKL consoles; real and handover
  * @atomic_seq:		Sequence for record tracking (32bit only)
  * @thread_pbufs:	Pointer to thread private buffer
+ * @write_atomic:	Write callback for atomic context
  * @pcpu_data:		Pointer to percpu context data
  */
 struct console {
@@ -338,6 +343,9 @@ struct console {
 	atomic_t		__private atomic_seq;
 #endif
 	struct printk_buffers	*thread_pbufs;
+
+	bool (*write_atomic)(struct console *con, struct cons_write_context *wctxt);
+
 	struct cons_context_data	__percpu *pcpu_data;
 };
 
Index: linux-6.3.0-rt11/kernel/printk/internal.h
===================================================================
--- linux-6.3.0-rt11.orig/kernel/printk/internal.h
+++ linux-6.3.0-rt11/kernel/printk/internal.h
@@ -133,3 +133,13 @@ struct printk_message {
 struct cons_context_data {
 	struct printk_buffers		pbufs;
 };
+
+bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
+			     bool is_extended, bool may_supress);
+
+#ifdef CONFIG_PRINTK
+
+void console_prepend_dropped(struct printk_message *pmsg,
+			     unsigned long dropped);
+
+#endif
Index: linux-6.3.0-rt11/kernel/printk/printk.c
===================================================================
--- linux-6.3.0-rt11.orig/kernel/printk/printk.c
+++ linux-6.3.0-rt11/kernel/printk/printk.c
@@ -711,9 +711,6 @@ out:
 	return len;
 }
 
-static bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
-				    bool is_extended, bool may_supress);
-
 /* /dev/kmsg - userspace message inject/listen interface */
 struct devkmsg_user {
 	atomic64_t seq;
@@ -2782,7 +2779,7 @@ static void __console_unlock(void)
  * If @pmsg->pbufs->outbuf is modified, @pmsg->outbuf_len is updated.
  */
 #ifdef CONFIG_PRINTK
-static void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped)
+void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped)
 {
 	struct printk_buffers *pbufs = pmsg->pbufs;
 	const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf);
@@ -2814,7 +2811,8 @@ static void console_prepend_dropped(stru
 	pmsg->outbuf_len += len;
 }
 #else
-#define console_prepend_dropped(pmsg, dropped)
+static inline void console_prepend_dropped(struct printk_message *pmsg,
+					   unsigned long dropped) { }
 #endif /* CONFIG_PRINTK */
 
 /*
@@ -2836,8 +2834,8 @@ static void console_prepend_dropped(stru
  * of @pmsg are valid. (See the documentation of struct printk_message
  * for information about the @pmsg fields.)
  */
-static bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
-				    bool is_extended, bool may_suppress)
+bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
+			     bool is_extended, bool may_suppress)
 {
 	static int panic_console_dropped;
 
Index: linux-6.3.0-rt11/kernel/printk/printk_nobkl.c
===================================================================
--- linux-6.3.0-rt11.orig/kernel/printk/printk_nobkl.c
+++ linux-6.3.0-rt11/kernel/printk/printk_nobkl.c
@@ -317,7 +317,7 @@ static void cons_context_set_seq(struct
  * invalid. Caller has to reacquire the console.
  */
 #ifdef CONFIG_64BIT
-static bool __maybe_unused cons_seq_try_update(struct cons_context *ctxt)
+static bool cons_seq_try_update(struct cons_context *ctxt)
 {
 	struct console *con = ctxt->console;
 	struct cons_state old;
@@ -346,7 +346,7 @@ static bool __maybe_unused cons_seq_try_
 }
 #else
 static bool cons_release(struct cons_context *ctxt);
-static bool __maybe_unused cons_seq_try_update(struct cons_context *ctxt)
+static bool cons_seq_try_update(struct cons_context *ctxt)
 {
 	struct console *con = ctxt->console;
 	struct cons_state state;
@@ -1090,6 +1090,123 @@ bool console_exit_unsafe(struct cons_wri
 EXPORT_SYMBOL_GPL(console_exit_unsafe);
 
 /**
+ * cons_get_record - Fill the buffer with the next pending ringbuffer record
+ * @wctxt:	The write context which will be handed to the write function
+ *
+ * Returns:	True if there are records available. If the next record should
+ *		be printed, the output buffer is filled and @wctxt->outbuf
+ *		points to the text to print. If @wctxt->outbuf is NULL after
+ *		the call, the record should not be printed but the caller must
+ *		still update the console sequence number.
+ *
+ *		False means that there are no pending records anymore and the
+ *		printing can stop.
+ */
+static bool cons_get_record(struct cons_write_context *wctxt)
+{
+	struct cons_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+	struct console *con = ctxt->console;
+	bool is_extended = console_srcu_read_flags(con) & CON_EXTENDED;
+	struct printk_message pmsg = {
+		.pbufs = ctxt->pbufs,
+	};
+
+	if (!printk_get_next_message(&pmsg, ctxt->newseq, is_extended, true))
+		return false;
+
+	ctxt->newseq = pmsg.seq;
+	ctxt->dropped += pmsg.dropped;
+
+	if (pmsg.outbuf_len == 0) {
+		wctxt->outbuf = NULL;
+	} else {
+		if (ctxt->dropped && !is_extended)
+			console_prepend_dropped(&pmsg, ctxt->dropped);
+		wctxt->outbuf = &pmsg.pbufs->outbuf[0];
+	}
+
+	wctxt->len = pmsg.outbuf_len;
+
+	return true;
+}
+
+/**
+ * cons_emit_record - Emit record in the acquired context
+ * @wctxt:	The write context that will be handed to the write function
+ *
+ * Returns:	False if the operation was aborted (takeover or handover).
+ *		True otherwise
+ *
+ * When false is returned, the caller is not allowed to touch console state.
+ * The console is owned by someone else. If the caller wants to print more
+ * it has to reacquire the console first.
+ *
+ * When true is returned, @wctxt->ctxt.backlog indicates whether there are
+ * still records pending in the ringbuffer,
+ */
+static int __maybe_unused cons_emit_record(struct cons_write_context *wctxt)
+{
+	struct cons_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+	struct console *con = ctxt->console;
+	bool done = false;
+
+	/*
+	 * @con->dropped is not protected in case of hostile takeovers so
+	 * the update below is racy. Annotate it accordingly.
+	 */
+	ctxt->dropped = data_race(READ_ONCE(con->dropped));
+
+	/* Fill the output buffer with the next record */
+	ctxt->backlog = cons_get_record(wctxt);
+	if (!ctxt->backlog)
+		return true;
+
+	/* Safety point. Don't touch state in case of takeover */
+	if (!console_can_proceed(wctxt))
+		return false;
+
+	/* Counterpart to the read above */
+	WRITE_ONCE(con->dropped, ctxt->dropped);
+
+	/*
+	 * In case of skipped records, Update sequence state in @con.
+	 */
+	if (!wctxt->outbuf)
+		goto update;
+
+	/* Tell the driver about potential unsafe state */
+	wctxt->unsafe = ctxt->state.unsafe;
+
+	if (!ctxt->thread && con->write_atomic) {
+		done = con->write_atomic(con, wctxt);
+	} else {
+		cons_release(ctxt);
+		WARN_ON_ONCE(1);
+		return false;
+	}
+
+	/* If not done, the write was aborted due to takeover */
+	if (!done)
+		return false;
+
+	/* If there was a dropped message, it has now been output. */
+	if (ctxt->dropped) {
+		ctxt->dropped = 0;
+		/* Counterpart to the read above */
+		WRITE_ONCE(con->dropped, ctxt->dropped);
+	}
+update:
+	ctxt->newseq++;
+	/*
+	 * The sequence update attempt is not part of console_release()
+	 * because in panic situations the console is not released by
+	 * the panic CPU until all records are written. On 32bit the
+	 * sequence is separate from state anyway.
+	 */
+	return cons_seq_try_update(ctxt);
+}
+
+/**
  * cons_nobkl_init - Initialize the NOBKL console specific data
  * @con:	Console to initialize
  *