* [PATCH] selftests/livepatch: better synchronize test_klp_callbacks_busy
@ 2022-05-18 17:34 Joe Lawrence
2022-05-18 18:18 ` Joe Lawrence
2022-05-23 15:28 ` Petr Mladek
0 siblings, 2 replies; 3+ messages in thread
From: Joe Lawrence @ 2022-05-18 17:34 UTC (permalink / raw)
To: live-patching; +Cc: Joe Lawrence, Miroslav Benes, Petr Mladek, Yannick Cote
The test_klp_callbacks_busy module conditionally blocks a future
livepatch transition by busy waiting inside its workqueue function,
busymod_work_func(). After scheduling this work, a test livepatch is
loaded, introducing the transition under test.
Both events are marked in the kernel log for later verification, but
there is no synchronization to ensure that busymod_work_func() logs its
function entry message before subsequent selftest commands log their own
messages. This can lead to a rare test failure due to unexpected
ordering like:
--- expected
+++ result
@@ -1,7 +1,7 @@
% modprobe test_klp_callbacks_busy block_transition=Y
test_klp_callbacks_busy: test_klp_callbacks_busy_init
-test_klp_callbacks_busy: busymod_work_func enter
% modprobe test_klp_callbacks_demo
+test_klp_callbacks_busy: busymod_work_func enter
livepatch: enabling patch 'test_klp_callbacks_demo'
livepatch: 'test_klp_callbacks_demo': initializing patching transition
test_klp_callbacks_demo: pre_patch_callback: vmlinux
Force the module init function to wait until busymod_work_func() has
started (and logged its message), before exiting to the next selftest
steps.
Fixes: 547840bd5ae5 ("selftests/livepatch: simplify test-klp-callbacks busy target tests")
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
---
Hi Petr, I remember you discouraged against a completion variable the
first time around [1], but is there any better way with the workqueue
API to ensure our "enter" message gets logged first? Or should we just
drop the msg altogether to avoid the situation? I don't think it's
absolutely necessary for the tests.
[1] https://lore.kernel.org/live-patching/20200602081654.GI27273@linux-b0ei/
lib/livepatch/test_klp_callbacks_busy.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/lib/livepatch/test_klp_callbacks_busy.c b/lib/livepatch/test_klp_callbacks_busy.c
index 7ac845f65be5..eb502b2bb3ef 100644
--- a/lib/livepatch/test_klp_callbacks_busy.c
+++ b/lib/livepatch/test_klp_callbacks_busy.c
@@ -16,10 +16,16 @@ MODULE_PARM_DESC(block_transition, "block_transition (default=false)");
static void busymod_work_func(struct work_struct *work);
static DECLARE_WORK(work, busymod_work_func);
+static DECLARE_COMPLETION(busymod_work_started);
static void busymod_work_func(struct work_struct *work)
{
+ /*
+ * Hold the init function from exiting until we've started and
+ * announced our appearence in the kernel log.
+ */
pr_info("%s enter\n", __func__);
+ complete(&busymod_work_started);
while (READ_ONCE(block_transition)) {
/*
@@ -36,6 +42,7 @@ static int test_klp_callbacks_busy_init(void)
{
pr_info("%s\n", __func__);
schedule_work(&work);
+ wait_for_completion(&busymod_work_started);
if (!block_transition) {
/*
--
2.26.3
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] selftests/livepatch: better synchronize test_klp_callbacks_busy
2022-05-18 17:34 [PATCH] selftests/livepatch: better synchronize test_klp_callbacks_busy Joe Lawrence
@ 2022-05-18 18:18 ` Joe Lawrence
2022-05-23 15:28 ` Petr Mladek
1 sibling, 0 replies; 3+ messages in thread
From: Joe Lawrence @ 2022-05-18 18:18 UTC (permalink / raw)
To: live-patching; +Cc: Miroslav Benes, Petr Mladek, Yannick Cote
On Wed, May 18, 2022 at 01:34:24PM -0400, Joe Lawrence wrote:
> The test_klp_callbacks_busy module conditionally blocks a future
> livepatch transition by busy waiting inside its workqueue function,
> busymod_work_func(). After scheduling this work, a test livepatch is
> loaded, introducing the transition under test.
>
> Both events are marked in the kernel log for later verification, but
> there is no synchronization to ensure that busymod_work_func() logs its
> function entry message before subsequent selftest commands log their own
> messages. This can lead to a rare test failure due to unexpected
> ordering like:
>
> --- expected
> +++ result
> @@ -1,7 +1,7 @@
> % modprobe test_klp_callbacks_busy block_transition=Y
> test_klp_callbacks_busy: test_klp_callbacks_busy_init
> -test_klp_callbacks_busy: busymod_work_func enter
> % modprobe test_klp_callbacks_demo
> +test_klp_callbacks_busy: busymod_work_func enter
> livepatch: enabling patch 'test_klp_callbacks_demo'
> livepatch: 'test_klp_callbacks_demo': initializing patching transition
> test_klp_callbacks_demo: pre_patch_callback: vmlinux
>
> Force the module init function to wait until busymod_work_func() has
> started (and logged its message), before exiting to the next selftest
> steps.
>
> Fixes: 547840bd5ae5 ("selftests/livepatch: simplify test-klp-callbacks busy target tests")
> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
> ---
>
> Hi Petr, I remember you discouraged against a completion variable the
> first time around [1], but is there any better way with the workqueue
> API to ensure our "enter" message gets logged first? Or should we just
> drop the msg altogether to avoid the situation? I don't think it's
> absolutely necessary for the tests.
>
> [1] https://lore.kernel.org/live-patching/20200602081654.GI27273@linux-b0ei/
>
> lib/livepatch/test_klp_callbacks_busy.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/lib/livepatch/test_klp_callbacks_busy.c b/lib/livepatch/test_klp_callbacks_busy.c
> index 7ac845f65be5..eb502b2bb3ef 100644
> --- a/lib/livepatch/test_klp_callbacks_busy.c
> +++ b/lib/livepatch/test_klp_callbacks_busy.c
> @@ -16,10 +16,16 @@ MODULE_PARM_DESC(block_transition, "block_transition (default=false)");
>
> static void busymod_work_func(struct work_struct *work);
> static DECLARE_WORK(work, busymod_work_func);
> +static DECLARE_COMPLETION(busymod_work_started);
>
> static void busymod_work_func(struct work_struct *work)
> {
> + /*
> + * Hold the init function from exiting until we've started and
> + * announced our appearence in the kernel log.
> + */
> pr_info("%s enter\n", __func__);
> + complete(&busymod_work_started);
>
> while (READ_ONCE(block_transition)) {
> /*
> @@ -36,6 +42,7 @@ static int test_klp_callbacks_busy_init(void)
> {
> pr_info("%s\n", __func__);
> schedule_work(&work);
> + wait_for_completion(&busymod_work_started);
>
> if (!block_transition) {
> /*
> --
> 2.26.3
>
[ fixup ycote's email address, sorry Yannick :D ]
-- Joe
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] selftests/livepatch: better synchronize test_klp_callbacks_busy
2022-05-18 17:34 [PATCH] selftests/livepatch: better synchronize test_klp_callbacks_busy Joe Lawrence
2022-05-18 18:18 ` Joe Lawrence
@ 2022-05-23 15:28 ` Petr Mladek
1 sibling, 0 replies; 3+ messages in thread
From: Petr Mladek @ 2022-05-23 15:28 UTC (permalink / raw)
To: Joe Lawrence; +Cc: live-patching, Miroslav Benes, Yannick Cote
On Wed 2022-05-18 13:34:24, Joe Lawrence wrote:
> The test_klp_callbacks_busy module conditionally blocks a future
> livepatch transition by busy waiting inside its workqueue function,
> busymod_work_func(). After scheduling this work, a test livepatch is
> loaded, introducing the transition under test.
>
> Both events are marked in the kernel log for later verification, but
> there is no synchronization to ensure that busymod_work_func() logs its
> function entry message before subsequent selftest commands log their own
> messages. This can lead to a rare test failure due to unexpected
> ordering like:
>
> --- expected
> +++ result
> @@ -1,7 +1,7 @@
> % modprobe test_klp_callbacks_busy block_transition=Y
> test_klp_callbacks_busy: test_klp_callbacks_busy_init
> -test_klp_callbacks_busy: busymod_work_func enter
> % modprobe test_klp_callbacks_demo
> +test_klp_callbacks_busy: busymod_work_func enter
> livepatch: enabling patch 'test_klp_callbacks_demo'
> livepatch: 'test_klp_callbacks_demo': initializing patching transition
> test_klp_callbacks_demo: pre_patch_callback: vmlinux
>
> Force the module init function to wait until busymod_work_func() has
> started (and logged its message), before exiting to the next selftest
> steps.
>
> Fixes: 547840bd5ae5 ("selftests/livepatch: simplify test-klp-callbacks busy target tests")
> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
> ---
>
> Hi Petr, I remember you discouraged against a completion variable the
> first time around [1], but is there any better way with the workqueue
> API to ensure our "enter" message gets logged first?
I think that the code was more complicated at that time.
Or I have got used to it ;-)
> Or should we just drop the msg altogether to avoid the situation?
> I don't think it's absolutely necessary for the tests.
IMHO, the message helps to make sure that the timing is correct.
I would keep it.
> [1] https://lore.kernel.org/live-patching/20200602081654.GI27273@linux-b0ei/
>
> lib/livepatch/test_klp_callbacks_busy.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/lib/livepatch/test_klp_callbacks_busy.c b/lib/livepatch/test_klp_callbacks_busy.c
> index 7ac845f65be5..eb502b2bb3ef 100644
> --- a/lib/livepatch/test_klp_callbacks_busy.c
> +++ b/lib/livepatch/test_klp_callbacks_busy.c
> @@ -16,10 +16,16 @@ MODULE_PARM_DESC(block_transition, "block_transition (default=false)");
>
> static void busymod_work_func(struct work_struct *work);
> static DECLARE_WORK(work, busymod_work_func);
> +static DECLARE_COMPLETION(busymod_work_started);
>
> static void busymod_work_func(struct work_struct *work)
> {
> + /*
> + * Hold the init function from exiting until we've started and
> + * announced our appearence in the kernel log.
> + */
This message would make more sense above the wait_for_completion().
The wait function holds the init function. I would remove the comment here.
> pr_info("%s enter\n", __func__);
> + complete(&busymod_work_started);
>
> while (READ_ONCE(block_transition)) {
> /*
> @@ -36,6 +42,7 @@ static int test_klp_callbacks_busy_init(void)
> {
> pr_info("%s\n", __func__);
> schedule_work(&work);
Instead, I would add here something like:
/*
* Hold the init function from exiting until the message
* about entering the busy loop is printed.
*/
> + wait_for_completion(&busymod_work_started);
>
> if (!block_transition) {
> /*
> --
> 2.26.3
With the updated message:
Reviewed-by: Petr Mladek <pmladek@suse.com>
Best Regards,
Petr
PS: I am sorry for the late review. I have busy times.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2022-05-18 17:34 [PATCH] selftests/livepatch: better synchronize test_klp_callbacks_busy Joe Lawrence
2022-05-18 18:18 ` Joe Lawrence
2022-05-23 15:28 ` Petr Mladek
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