[PATCH rcu 1/2] torture: Optionally flush printk() buffers before powering off

[Paul E. McKenney]

The rcutorture test suite produces quite a bit of console output at
the end of a test.  This means that the new-in-2022 printk() kthreads
are likely to be in the process of flushing output at the time of the
torture_shutdown() function's call to kernel_power_off().  Normally,
rcutorture relies on printk() to flush any pending output upon shutdown,
the better to detect bugs in this area, for example, the one introduced
by 8e274732115f ("printk: extend console_lock for per-console locking").
However, once such a bug is detected and reported, it is necessary to
test the rest of the system, without noise from the already-reported bug.

This commit therefore adds a torture.printk_shutdown_bug_workaround
kernel parameter, which causes torture_shutdown() to invoke pr_flush(),
and print an informative message on the console, immediately before
invoking kernel_power_off().  When this kernel parameter is not specified,
it is up to printk() to flush its own buffers.

Suggested-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
 kernel/torture.c                                | 7 +++++++
 2 files changed, 13 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d7f30902fda02..ba1b8776aab83 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6143,6 +6143,12 @@
 			are running concurrently, especially on systems
 			with rotating-rust storage.
 
+	torture.printk_shutdown_bug_workaround= [KNL]
+			Execute pr_flush(1000, true) just before invoking
+			kernel_power_off() to work around any bugs that
+			might prevent printk() from flushing its buffers
+			at shutdown time.
+
 	torture.verbose_sleep_frequency= [KNL]
 			Specifies how many verbose printk()s should be
 			emitted between each sleep.  The default of zero
diff --git a/kernel/torture.c b/kernel/torture.c
index 789aeb0e1159c..7cd2016b02076 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -48,6 +48,9 @@ module_param(disable_onoff_at_boot, bool, 0444);
 static bool ftrace_dump_at_shutdown;
 module_param(ftrace_dump_at_shutdown, bool, 0444);
 
+static bool printk_shutdown_bug_workaround;
+module_param(printk_shutdown_bug_workaround, bool, 0444);
+
 static int verbose_sleep_frequency;
 module_param(verbose_sleep_frequency, int, 0444);
 
@@ -651,6 +654,10 @@ static int torture_shutdown(void *arg)
 		VERBOSE_TOROUT_STRING("No torture_shutdown_hook(), skipping.");
 	if (ftrace_dump_at_shutdown)
 		rcu_ftrace_dump(DUMP_ALL);
+	if (printk_shutdown_bug_workaround) {
+		pr_info("%s: Flushing printk() buffers at power-down time.\n", __func__);
+		pr_flush(1000, true);
+	}
 	kernel_power_off();	/* Shut down the system. */
 	return 0;
 }
-- 
2.31.1.189.g2e36527f23