2 files changed, 96 insertions, 35 deletions

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 6080d289a342..c9bb0cd86372 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -92,3 +92,19 @@ struct printk_buffers {
 	char	outbuf[CONSOLE_EXT_LOG_MAX];
 	char	scratchbuf[LOG_LINE_MAX];
 };
+
+/**
+ * struct printk_message - Container for a prepared printk message.
+ * @pbufs:	printk buffers used to prepare the message.
+ * @outbuf_len:	The length of prepared text in @pbufs->outbuf to output. This
+ *		does not count the terminator. A value of 0 means there is
+ *		nothing to output and this record should be skipped.
+ * @seq:	The sequence number of the record used for @pbufs->outbuf.
+ * @dropped:	The number of dropped records from reading @seq.
+ */
+struct printk_message {
+	struct printk_buffers	*pbufs;
+	unsigned int		outbuf_len;
+	u64			seq;
+	unsigned long		dropped;
+};
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index bc5d4574c459..6e2a6d5548e9 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2725,35 +2725,35 @@ static void __console_unlock(void)
 }
 
 /*
- * Print one record for the given console. The record printed is whatever
- * record is the next available record for the given console.
+ * Read and format the specified record (or a later record if the specified
+ * record is not available).
  *
- * @handover will be set to true if a printk waiter has taken over the
- * console_lock, in which case the caller is no longer holding both the
- * console_lock and the SRCU read lock. Otherwise it is set to false.
+ * @pmsg will contain the formatted result. @pmsg->pbufs must point to a
+ * struct printk_buffers.
  *
- * @cookie is the cookie from the SRCU read lock.
+ * @seq is the record to read and format. If it is not available, the next
+ * valid record is read.
  *
- * Returns false if the given console has no next record to print, otherwise
- * true.
+ * @is_extended specifies if the message should be formatted for extended
+ * console output.
  *
- * Requires the console_lock and the SRCU read lock.
+ * Returns false if no record is available. Otherwise true and all fields
+ * of @pmsg are valid. (See the documentation of struct printk_message
+ * for information about the @pmsg fields.)
  */
-static bool console_emit_next_record(struct console *con, bool *handover, int cookie)
+static bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
+				    bool is_extended)
 {
-	static char dropped_text[DROPPED_TEXT_MAX];
-	static struct printk_buffers pbufs;
 	static int panic_console_dropped;
 
-	bool is_extended = console_srcu_read_flags(con) & CON_EXTENDED;
-	const size_t scratchbuf_sz = sizeof(pbufs.scratchbuf);
-	const size_t outbuf_sz = sizeof(pbufs.outbuf);
-	char *scratchbuf = &pbufs.scratchbuf[0];
-	char *outbuf = &pbufs.outbuf[0];
+	struct printk_buffers *pbufs = pmsg->pbufs;
+	const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf);
+	const size_t outbuf_sz = sizeof(pbufs->outbuf);
+	char *scratchbuf = &pbufs->scratchbuf[0];
+	char *outbuf = &pbufs->outbuf[0];
 	struct printk_info info;
 	struct printk_record r;
-	unsigned long flags;
-	size_t len;
+	size_t len = 0;
 
 	/*
 	 * Formatting extended messages requires a separate buffer, so use the
@@ -2767,25 +2767,26 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
 	else
 		prb_rec_init_rd(&r, &info, outbuf, outbuf_sz);
 
-	*handover = false;
-
-	if (!prb_read_valid(prb, con->seq, &r))
+	if (!prb_read_valid(prb, seq, &r))
 		return false;
 
-	if (con->seq != r.info->seq) {
-		con->dropped += r.info->seq - con->seq;
-		con->seq = r.info->seq;
-		if (panic_in_progress() && panic_console_dropped++ > 10) {
-			suppress_panic_printk = 1;
-			pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n");
-		}
+	pmsg->seq = r.info->seq;
+	pmsg->dropped = r.info->seq - seq;
+
+	/*
+	 * Check for dropped messages in panic here so that printk
+	 * suppression can occur as early as possible if necessary.
+	 */
+	if (pmsg->dropped &&
+	    panic_in_progress() &&
+	    panic_console_dropped++ > 10) {
+		suppress_panic_printk = 1;
+		pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n");
 	}
 
 	/* Skip record that has level above the console loglevel. */
-	if (suppress_message_printing(r.info->level)) {
-		con->seq++;
-		goto skip;
-	}
+	if (suppress_message_printing(r.info->level))
+		goto out;
 
 	if (is_extended) {
 		len = info_print_ext_header(outbuf, outbuf_sz, r.info);
@@ -2794,6 +2795,50 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
 	} else {
 		len = record_print_text(&r, console_msg_format & MSG_FORMAT_SYSLOG, printk_time);
 	}
+out:
+	pmsg->outbuf_len = len;
+	return true;
+}
+
+/*
+ * Print one record for the given console. The record printed is whatever
+ * record is the next available record for the given console.
+ *
+ * @handover will be set to true if a printk waiter has taken over the
+ * console_lock, in which case the caller is no longer holding both the
+ * console_lock and the SRCU read lock. Otherwise it is set to false.
+ *
+ * @cookie is the cookie from the SRCU read lock.
+ *
+ * Returns false if the given console has no next record to print, otherwise
+ * true.
+ *
+ * Requires the console_lock and the SRCU read lock.
+ */
+static bool console_emit_next_record(struct console *con, bool *handover, int cookie)
+{
+	static char dropped_text[DROPPED_TEXT_MAX];
+	static struct printk_buffers pbufs;
+
+	bool is_extended = console_srcu_read_flags(con) & CON_EXTENDED;
+	char *outbuf = &pbufs.outbuf[0];
+	struct printk_message pmsg = {
+		.pbufs = &pbufs,
+	};
+	unsigned long flags;
+
+	*handover = false;
+
+	if (!printk_get_next_message(&pmsg, con->seq, is_extended))
+		return false;
+
+	con->dropped += pmsg.dropped;
+
+	/* Skip messages of formatted length 0. */
+	if (pmsg.outbuf_len == 0) {
+		con->seq = pmsg.seq + 1;
+		goto skip;
+	}
 
 	/*
 	 * While actively printing out messages, if another printk()
@@ -2809,11 +2854,11 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
 	console_lock_spinning_enable();
 
 	stop_critical_timings();	/* don't trace print latency */
-	call_console_driver(con, outbuf, len,
+	call_console_driver(con, outbuf, pmsg.outbuf_len,
 			    is_extended ? NULL : dropped_text);
 	start_critical_timings();
 
-	con->seq++;
+	con->seq = pmsg.seq + 1;
 
 	*handover = console_lock_spinning_disable_and_check(cookie);
 	printk_safe_exit_irqrestore(flags);