Patch "printk: Consolidate console deferred printing" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    printk: Consolidate console deferred printing

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     printk-consolidate-console-deferred-printing.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 84c248b03e6537eba02569b796651cd9656dc9ba
Author: John Ogness <john.ogness@xxxxxxxxxxxxx>
Date:   Mon Jul 17 21:52:05 2023 +0206

    printk: Consolidate console deferred printing
    
    [ Upstream commit 696ffaf50e1f8dbc66223ff614473f945f5fb8d8 ]
    
    Printing to consoles can be deferred for several reasons:
    
    - explicitly with printk_deferred()
    - printk() in NMI context
    - recursive printk() calls
    
    The current implementation is not consistent. For printk_deferred(),
    irq work is scheduled twice. For NMI und recursive, panic CPU
    suppression and caller delays are not properly enforced.
    
    Correct these inconsistencies by consolidating the deferred printing
    code so that vprintk_deferred() is the top-level function for
    deferred printing and vprintk_emit() will perform whichever irq_work
    queueing is appropriate.
    
    Also add kerneldoc for wake_up_klogd() and defer_console_output() to
    clarify their differences and appropriate usage.
    
    Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
    Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
    Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
    Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
    Link: https://lore.kernel.org/r/20230717194607.145135-6-john.ogness@xxxxxxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8d856b7c2e5af..8b110b245d92c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2269,7 +2269,11 @@ asmlinkage int vprintk_emit(int facility, int level,
 		preempt_enable();
 	}
 
-	wake_up_klogd();
+	if (in_sched)
+		defer_console_output();
+	else
+		wake_up_klogd();
+
 	return printed_len;
 }
 EXPORT_SYMBOL(vprintk_emit);
@@ -3277,11 +3281,33 @@ static void __wake_up_klogd(int val)
 	preempt_enable();
 }
 
+/**
+ * wake_up_klogd - Wake kernel logging daemon
+ *
+ * Use this function when new records have been added to the ringbuffer
+ * and the console printing of those records has already occurred or is
+ * known to be handled by some other context. This function will only
+ * wake the logging daemon.
+ *
+ * Context: Any context.
+ */
 void wake_up_klogd(void)
 {
 	__wake_up_klogd(PRINTK_PENDING_WAKEUP);
 }
 
+/**
+ * defer_console_output - Wake kernel logging daemon and trigger
+ *	console printing in a deferred context
+ *
+ * Use this function when new records have been added to the ringbuffer,
+ * this context is responsible for console printing those records, but
+ * the current context is not allowed to perform the console printing.
+ * Trigger an irq_work context to perform the console printing. This
+ * function also wakes the logging daemon.
+ *
+ * Context: Any context.
+ */
 void defer_console_output(void)
 {
 	/*
@@ -3298,12 +3324,7 @@ void printk_trigger_flush(void)
 
 int vprintk_deferred(const char *fmt, va_list args)
 {
-	int r;
-
-	r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args);
-	defer_console_output();
-
-	return r;
+	return vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args);
 }
 
 int _printk_deferred(const char *fmt, ...)
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index ef0f9a2044da1..6d10927a07d83 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -38,13 +38,8 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	 * Use the main logbuf even in NMI. But avoid calling console
 	 * drivers that might have their own locks.
 	 */
-	if (this_cpu_read(printk_context) || in_nmi()) {
-		int len;
-
-		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
-		defer_console_output();
-		return len;
-	}
+	if (this_cpu_read(printk_context) || in_nmi())
+		return vprintk_deferred(fmt, args);
 
 	/* No obstacles. */
 	return vprintk_default(fmt, args);