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

From: Paul E. McKenney
Date: Wed Aug 31 2022 - 14:26:09 EST


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