[v3,06/15] net: kunit: Suppress lock warning noise at end of dev_addr_lists tests

Commit Message

Guenter Roeck April 3, 2024, 1:19 p.m. UTC

dev_addr_lists_test generates lock warning noise at the end of tests
if lock debugging is enabled. There are two sets of warnings.

WARNING: CPU: 0 PID: 689 at kernel/locking/mutex.c:923 __mutex_unlock_slowpath.constprop.0+0x13c/0x368
DEBUG_LOCKS_WARN_ON(__owner_task(owner) != __get_current())

WARNING: kunit_try_catch/1336 still has locks held!

KUnit test cleanup is not guaranteed to run in the same thread as the test
itself. For this test, this means that rtnl_lock() and rtnl_unlock() may
be called from different threads. This triggers the warnings.
Suppress the warnings because they are irrelevant for the test and just
confusing and distracting.

The first warning can be suppressed by using START_SUPPRESSED_WARNING()
and END_SUPPRESSED_WARNING() around the call to rtnl_unlock(). To suppress
the second warning, it is necessary to set debug_locks_silent while the
rtnl lock is held.

Tested-by: Linux Kernel Functional Testing <[e-mail]>
Cc: David Gow <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Acked-by: Dan Carpenter <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
---
v2:
- Rebased to v6.9-rc1
- Added Tested-by:, Acked-by:, and Reviewed-by: tags
v3:
- Rebased to v6.9-rc2

 net/core/dev_addr_lists_test.c | 6 ++++++
 1 file changed, 6 insertions(+)

Comments

Jakub Kicinski April 4, 2024, 1:34 a.m. UTC | #1

On Wed,  3 Apr 2024 06:19:27 -0700 Guenter Roeck wrote:
> dev_addr_lists_test generates lock warning noise at the end of tests
> if lock debugging is enabled. There are two sets of warnings.
> 
> WARNING: CPU: 0 PID: 689 at kernel/locking/mutex.c:923 __mutex_unlock_slowpath.constprop.0+0x13c/0x368
> DEBUG_LOCKS_WARN_ON(__owner_task(owner) != __get_current())
> 
> WARNING: kunit_try_catch/1336 still has locks held!
> 
> KUnit test cleanup is not guaranteed to run in the same thread as the test
> itself. For this test, this means that rtnl_lock() and rtnl_unlock() may
> be called from different threads. This triggers the warnings.
> Suppress the warnings because they are irrelevant for the test and just
> confusing and distracting.
> 
> The first warning can be suppressed by using START_SUPPRESSED_WARNING()
> and END_SUPPRESSED_WARNING() around the call to rtnl_unlock(). To suppress
> the second warning, it is necessary to set debug_locks_silent while the
> rtnl lock is held.

Is it okay if I move the locking into the tests, instead?
It's only 4 lines more and no magic required, seems to work fine.

Guenter Roeck April 8, 2024, 4:34 p.m. UTC | #2

On Wed, Apr 03, 2024 at 06:34:12PM -0700, Jakub Kicinski wrote:
> On Wed,  3 Apr 2024 06:19:27 -0700 Guenter Roeck wrote:
> > dev_addr_lists_test generates lock warning noise at the end of tests
> > if lock debugging is enabled. There are two sets of warnings.
> > 
> > WARNING: CPU: 0 PID: 689 at kernel/locking/mutex.c:923 __mutex_unlock_slowpath.constprop.0+0x13c/0x368
> > DEBUG_LOCKS_WARN_ON(__owner_task(owner) != __get_current())
> > 
> > WARNING: kunit_try_catch/1336 still has locks held!
> > 
> > KUnit test cleanup is not guaranteed to run in the same thread as the test
> > itself. For this test, this means that rtnl_lock() and rtnl_unlock() may
> > be called from different threads. This triggers the warnings.
> > Suppress the warnings because they are irrelevant for the test and just
> > confusing and distracting.
> > 
> > The first warning can be suppressed by using START_SUPPRESSED_WARNING()
> > and END_SUPPRESSED_WARNING() around the call to rtnl_unlock(). To suppress
> > the second warning, it is necessary to set debug_locks_silent while the
> > rtnl lock is held.
> 
> Is it okay if I move the locking into the tests, instead?
> It's only 4 lines more and no magic required, seems to work fine.

I don't think it is that simple. Unit tests run in a kernel thread
and exit immediately if a test fails. While the unit test code _looks_
sequential, that isn't really the case. Every instance of KUNIT_ASSERT_x
or KUNIT_FAIL() results in immediate kernel thread termination. If
that happens, any rtnl_unlock() in the failed function would not be
executed. I am not aware of an equivalent of atexit() for kernel threads
which would fix the problem. My understanding is that the kunit system
doesn't support an equivalent either, but at least sometimes executes
the exit function in a different thread context.

Guenter

Patch

diff --git a/net/core/dev_addr_lists_test.c b/net/core/dev_addr_lists_test.c
index 4dbd0dc6aea2..b427dd1a3c93 100644
--- a/net/core/dev_addr_lists_test.c
+++ b/net/core/dev_addr_lists_test.c
@@ -1,6 +1,7 @@ 
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #include <kunit/test.h>
+#include <linux/debug_locks.h>
 #include <linux/etherdevice.h>
 #include <linux/netdevice.h>
 #include <linux/rtnetlink.h>
@@ -49,6 +50,7 @@  static int dev_addr_test_init(struct kunit *test)
 		KUNIT_FAIL(test, "Can't register netdev %d", err);
 	}
 
+	debug_locks_silent = 1;
 	rtnl_lock();
 	return 0;
 }
@@ -56,8 +58,12 @@  static int dev_addr_test_init(struct kunit *test)
 static void dev_addr_test_exit(struct kunit *test)
 {
 	struct net_device *netdev = test->priv;
+	DEFINE_SUPPRESSED_WARNING(__mutex_unlock_slowpath);
 
+	START_SUPPRESSED_WARNING(__mutex_unlock_slowpath);
 	rtnl_unlock();
+	END_SUPPRESSED_WARNING(__mutex_unlock_slowpath);
+	debug_locks_silent = 0;
 	unregister_netdev(netdev);
 	free_netdev(netdev);
 }