Re: [PATCH v3] lkdtm: Add lockdep related crash tests

[Tzung-Bi Shih <tzungbi@kernel.org>]

On Wed, Mar 18, 2026 at 02:30:37PM +0900, Yunseong Kim wrote:
> Hi Tzung-Bi,
> 
> Thanks for the great work!
> 
> I'm very interested in this work as it will be helpful for verifying
> whether kernel crash dumps are correctly extracted when a deadlock is
> detected by lockdep. I really hope to see this merged.
> 
> I have a couple of questions regarding the organization of these tests:
> 
> Is the intent of this new lockdep.c specifically to validate the lockdep
> infrastructure itself rather than providing general deadlock.c code It
> feels like "deadlock" might be a broader category. For example, DEPT
> (Dependency Tracker) can track wait/event dependencies for PG_locked/writeback.
> I'm planning a follow-up contribution to cover these cases that DEPT can detect.

Yes, the lockdep.c is designed for lockdep; "deadlock" should be a
different set of issues (e.g., lockdep isn't only for deadlock).

> 
> Do you think it would be better to implement deadlock detection for LKDTM
> (like the ones DEPT handles) in a separate deadlock.c instead of
> lockdep.c? I'd love to hear your thoughts on the scope.

I think separate them would be easier to understand and review.

> 
> Best regards,
> Yunseong
> 
> On 12/15/25 4:25 PM, Tzung-Bi Shih wrote:
> > Introduce various lockdep related crash tests.
> > 
> > Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
> > ---
> > v3:
> > - Add some entries in tools/testing/selftests/lkdtm/tests.txt.
> > 
> > v2: https://lore.kernel.org/all/20251117033337.3730681-1-tzungbi@kernel.org/
> > - Fix "warning: suggest braces around empty body in an 'else' statement [-Wempty-body]"
> >   reported by 0day test robot.
> > 
> > v1: https://lore.kernel.org/lkml/20251114062535.1827309-1-tzungbi@kernel.org/T/#u
> > 
> >  drivers/misc/lkdtm/Makefile             |  1 +
> >  drivers/misc/lkdtm/core.c               |  1 +
> >  drivers/misc/lkdtm/lkdtm.h              |  1 +
> >  drivers/misc/lkdtm/lockdep.c            | 98 +++++++++++++++++++++++++
> >  tools/testing/selftests/lkdtm/tests.txt |  8 ++
> >  5 files changed, 109 insertions(+)
> >  create mode 100644 drivers/misc/lkdtm/lockdep.c
> > 
> > diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> > index 03ebe33185f9..830b71c8e6a0 100644
> > --- a/drivers/misc/lkdtm/Makefile
> > +++ b/drivers/misc/lkdtm/Makefile
> > @@ -11,6 +11,7 @@ lkdtm-$(CONFIG_LKDTM)		+= usercopy.o
> >  lkdtm-$(CONFIG_LKDTM)		+= kstack_erase.o
> >  lkdtm-$(CONFIG_LKDTM)		+= cfi.o
> >  lkdtm-$(CONFIG_LKDTM)		+= fortify.o
> > +lkdtm-$(CONFIG_LKDTM)		+= lockdep.o
> >  lkdtm-$(CONFIG_PPC_64S_HASH_MMU)	+= powerpc.o
> >  
> >  KASAN_SANITIZE_stackleak.o	:= n
> > diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> > index 5732fd59a227..43e91388940f 100644
> > --- a/drivers/misc/lkdtm/core.c
> > +++ b/drivers/misc/lkdtm/core.c
> > @@ -96,6 +96,7 @@ static const struct crashtype_category *crashtype_categories[] = {
> >  	&stackleak_crashtypes,
> >  	&cfi_crashtypes,
> >  	&fortify_crashtypes,
> > +	&lockdep_crashtypes,
> >  #ifdef CONFIG_PPC_64S_HASH_MMU
> >  	&powerpc_crashtypes,
> >  #endif
> > diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> > index 015e0484026b..d2d97e6f323e 100644
> > --- a/drivers/misc/lkdtm/lkdtm.h
> > +++ b/drivers/misc/lkdtm/lkdtm.h
> > @@ -84,6 +84,7 @@ extern struct crashtype_category usercopy_crashtypes;
> >  extern struct crashtype_category stackleak_crashtypes;
> >  extern struct crashtype_category cfi_crashtypes;
> >  extern struct crashtype_category fortify_crashtypes;
> > +extern struct crashtype_category lockdep_crashtypes;
> >  extern struct crashtype_category powerpc_crashtypes;
> >  
> >  /* Each category's init/exit routines. */
> > diff --git a/drivers/misc/lkdtm/lockdep.c b/drivers/misc/lkdtm/lockdep.c
> > new file mode 100644
> > index 000000000000..e029e9e60ce6
> > --- /dev/null
> > +++ b/drivers/misc/lkdtm/lockdep.c
> > @@ -0,0 +1,98 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Copyright 2025 Google LLC
> > + *
> > + * Tests related to lockdep warnings.
> > + */
> > +#include "lkdtm.h"
> > +#include <linux/cleanup.h>
> > +#include <linux/irqflags.h>
> > +#include <linux/mutex.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/srcu.h>
> > +
> > +static DEFINE_SPINLOCK(lock_A);
> > +static DEFINE_SPINLOCK(lock_B);
> > +
> > +/* For "WARNING: possible circular locking dependency detected". */
> > +static void lkdtm_LOCKDEP_CIRCULAR_LOCK(void)
> > +{
> > +	scoped_guard(spinlock, &lock_A)
> > +		scoped_guard(spinlock, &lock_B) {}
> > +	scoped_guard(spinlock, &lock_B)
> > +		scoped_guard(spinlock, &lock_A) {}
> > +}
> > +
> > +/* For "WARNING: possible recursive locking detected". */
> > +static void lkdtm_LOCKDEP_RECURSIVE_LOCK(void)
> > +{
> > +	guard(spinlock)(&lock_A);
> > +	guard(spinlock)(&lock_A);
> > +}
> > +
> > +/* For "WARNING: inconsistent lock state". */
> > +static void lkdtm_LOCKDEP_INCONSISTENT_LOCK(void)
> > +{
> > +	lockdep_softirq_enter();
> > +	scoped_guard(spinlock, &lock_A) {}
> > +	lockdep_softirq_exit();
> > +
> > +	scoped_guard(spinlock, &lock_A) {}
> > +}
> > +
> > +/* For "WARNING: Nested lock was not taken". */
> > +static void lkdtm_LOCKDEP_NESTED_LOCK_NOT_HELD(void)
> > +{
> > +	spin_lock_nest_lock(&lock_B, &lock_A);
> > +}
> > +
> > +/* For "WARNING: bad unlock balance detected!". */
> > +static void lkdtm_LOCKDEP_BAD_UNLOCK_BALANCE(void)
> > +{
> > +	spin_unlock(&lock_A);
> > +}
> > +
> > +/* For "WARNING: held lock freed!". */
> > +static void lkdtm_LOCKDEP_HELD_LOCK_FREED(void)
> > +{
> > +	spin_lock(&lock_A);
> > +	spin_lock_init(&lock_A);
> > +}
> > +
> > +/* For "WARNING: lock held when returning to user space!". */
> > +static void lkdtm_LOCKDEP_HELD_LOCK(void)
> > +{
> > +	spin_lock(&lock_A);
> > +}
> > +
> > +/* For "WARNING: suspicious RCU usage". */
> > +static void lkdtm_LOCKDEP_SUSPICIOUS_RCU(void)
> > +{
> > +	struct srcu_struct srcu;
> > +	void __rcu *res = NULL;
> > +	int idx;
> > +
> > +	init_srcu_struct(&srcu);
> > +
> > +	idx = srcu_read_lock(&srcu);
> > +	rcu_dereference(res);
> > +	srcu_read_unlock(&srcu, idx);
> > +
> > +	cleanup_srcu_struct(&srcu);
> > +}
> > +
> > +static struct crashtype crashtypes[] = {
> > +	CRASHTYPE(LOCKDEP_CIRCULAR_LOCK),
> > +	CRASHTYPE(LOCKDEP_RECURSIVE_LOCK),
> > +	CRASHTYPE(LOCKDEP_INCONSISTENT_LOCK),
> > +	CRASHTYPE(LOCKDEP_NESTED_LOCK_NOT_HELD),
> > +	CRASHTYPE(LOCKDEP_BAD_UNLOCK_BALANCE),
> > +	CRASHTYPE(LOCKDEP_HELD_LOCK_FREED),
> > +	CRASHTYPE(LOCKDEP_HELD_LOCK),
> > +	CRASHTYPE(LOCKDEP_SUSPICIOUS_RCU),
> > +};
> > +
> > +struct crashtype_category lockdep_crashtypes = {
> > +	.crashtypes = crashtypes,
> > +	.len	    = ARRAY_SIZE(crashtypes),
> > +};
> > diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
> > index cff124c1eddd..7b32dece3d3a 100644
> > --- a/tools/testing/selftests/lkdtm/tests.txt
> > +++ b/tools/testing/selftests/lkdtm/tests.txt
> > @@ -83,3 +83,11 @@ FORTIFY_STR_MEMBER detected buffer overflow
> >  FORTIFY_MEM_OBJECT detected buffer overflow
> >  FORTIFY_MEM_MEMBER detected field-spanning write
> >  PPC_SLB_MULTIHIT Recovered
> > +#LOCKDEP_CIRCULAR_LOCK Lockdep can only trigger once
> > +#LOCKDEP_RECURSIVE_LOCK Hangs the system
> > +#LOCKDEP_INCONSISTENT_LOCK Lockdep can only trigger once
> > +#LOCKDEP_NESTED_LOCK_NOT_HELD Cause a deadlock in subsequent lockdep tests
> > +#LOCKDEP_BAD_UNLOCK_BALANCE Lockdep can only trigger once
> > +#LOCKDEP_HELD_LOCK_FREED Lockdep can only trigger once
> > +#LOCKDEP_HELD_LOCK Cause a deadlock in subsequent lockdep tests
> > +#LOCKDEP_SUSPICIOUS_RCU Lockdep can only trigger once
>