[PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled

[PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled

[Bart Van Assche]

Commit a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer
in use") changed the behavior of lockdep_free_key_range() from
unconditionally zapping lock classes into only zapping lock classes if
debug_lock == true. Since the new behavior can cause cat /proc/lockdep to
crash due to a NULL pointer dereference, restore the pre-v5.1 behavior.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Waiman Long <longman@redhat.com>
Cc: shenghui <shhuiw@foxmail.com>
Reported-by: shenghui <shhuiw@foxmail.com>
Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use") # v5.1-rc1.
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 kernel/locking/lockdep.c | 23 ++++++-----------------
 1 file changed, 6 insertions(+), 17 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 34cdcbedda49..70480e4f8f5d 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4689,8 +4689,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
 		return;
 
 	raw_local_irq_save(flags);
-	if (!graph_lock())
-		goto out_irq;
+	arch_spin_lock(&lockdep_lock);
 
 	/* closed head */
 	pf = delayed_free.pf + (delayed_free.index ^ 1);
@@ -4702,8 +4701,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
 	 */
 	call_rcu_zapped(delayed_free.pf + delayed_free.index);
 
-	graph_unlock();
-out_irq:
+	arch_spin_unlock(&lockdep_lock);
 	raw_local_irq_restore(flags);
 }
 
@@ -4744,21 +4742,15 @@ static void lockdep_free_key_range_reg(void *start, unsigned long size)
 {
 	struct pending_free *pf;
 	unsigned long flags;
-	int locked;
 
 	init_data_structures_once();
 
 	raw_local_irq_save(flags);
-	locked = graph_lock();
-	if (!locked)
-		goto out_irq;
-
+	arch_spin_lock(&lockdep_lock);
 	pf = get_pending_free();
 	__lockdep_free_key_range(pf, start, size);
 	call_rcu_zapped(pf);
-
-	graph_unlock();
-out_irq:
+	arch_spin_unlock(&lockdep_lock);
 	raw_local_irq_restore(flags);
 
 	/*
@@ -4911,9 +4903,7 @@ void lockdep_unregister_key(struct lock_class_key *key)
 		return;
 
 	raw_local_irq_save(flags);
-	if (!graph_lock())
-		goto out_irq;
-
+	arch_spin_lock(&lockdep_lock);
 	pf = get_pending_free();
 	hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
 		if (k == key) {
@@ -4925,8 +4915,7 @@ void lockdep_unregister_key(struct lock_class_key *key)
 	WARN_ON_ONCE(!found);
 	__lockdep_free_key_range(pf, key, 1);
 	call_rcu_zapped(pf);
-	graph_unlock();
-out_irq:
+	arch_spin_unlock(&lockdep_lock);
 	raw_local_irq_restore(flags);
 
 	/* Wait until is_dynamic_key() has finished accessing k->hash_entry. */
-- 
2.21.0.196.g041f5ea1cf98

Re: [PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled

[Will Deacon]

On Tue, Mar 26, 2019 at 12:59:12PM -0700, Bart Van Assche wrote:
> Commit a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer
> in use") changed the behavior of lockdep_free_key_range() from
> unconditionally zapping lock classes into only zapping lock classes if
> debug_lock == true. Since the new behavior can cause cat /proc/lockdep to
> crash due to a NULL pointer dereference, restore the pre-v5.1 behavior.

Can you elaborate on this NULL dereference please, and why this patch fixes
it?

> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Waiman Long <longman@redhat.com>
> Cc: shenghui <shhuiw@foxmail.com>
> Reported-by: shenghui <shhuiw@foxmail.com>
> Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use") # v5.1-rc1.
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  kernel/locking/lockdep.c | 23 ++++++-----------------
>  1 file changed, 6 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 34cdcbedda49..70480e4f8f5d 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -4689,8 +4689,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
>  		return;
>  
>  	raw_local_irq_save(flags);
> -	if (!graph_lock())
> -		goto out_irq;
> +	arch_spin_lock(&lockdep_lock);

This also throws out the recursion counting. Is that ok?

Will

Re: [PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled

[Bart Van Assche]

On Wed, 2019-04-03 at 13:44 +0100, Will Deacon wrote:
> On Tue, Mar 26, 2019 at 12:59:12PM -0700, Bart Van Assche wrote:
> > Commit a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer
> > in use") changed the behavior of lockdep_free_key_range() from
> > unconditionally zapping lock classes into only zapping lock classes if
> > debug_lock == true. Since the new behavior can cause cat /proc/lockdep to
> > crash due to a NULL pointer dereference, restore the pre-v5.1 behavior.
> 
> Can you elaborate on this NULL dereference please, and why this patch fixes
> it?

Hi Will,

Not zapping lock classes if debug_lock == false leaves dangling pointers in
several lockdep datastructures, e.g. lock_class::name in the all_lock_classes
list. The shell command "cat /proc/lockdep" causes the kernel to iterate the
all_lock_classes list. Hence the "unable to handle kernel paging request"
issue that Shenghui encountered by running cat /proc/lockdep. Please let me
know if you would like me to repost this patch with a more detailed
description.

> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Cc: Waiman Long <longman@redhat.com>
> > Cc: shenghui <shhuiw@foxmail.com>
> > Reported-by: shenghui <shhuiw@foxmail.com>
> > Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use") # v5.1-rc1.
> > Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> > ---
> >  kernel/locking/lockdep.c | 23 ++++++-----------------
> >  1 file changed, 6 insertions(+), 17 deletions(-)
> > 
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index 34cdcbedda49..70480e4f8f5d 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -4689,8 +4689,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
> >  		return;
> >  
> >  	raw_local_irq_save(flags);
> > -	if (!graph_lock())
> > -		goto out_irq;
> > +	arch_spin_lock(&lockdep_lock);
> 
> This also throws out the recursion counting. Is that ok?

I think that that's OK. My understanding is that lockdep keeps track of
recursion to avoid that lockdep_lock is locked recursively. However, none
of the functions modified by this patch are called with that lock held.

Thanks,

Bart.

Re: [PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled

[Will Deacon]

On Wed, Apr 03, 2019 at 08:48:02AM -0700, Bart Van Assche wrote:
> On Wed, 2019-04-03 at 13:44 +0100, Will Deacon wrote:
> > On Tue, Mar 26, 2019 at 12:59:12PM -0700, Bart Van Assche wrote:
> > > Commit a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer
> > > in use") changed the behavior of lockdep_free_key_range() from
> > > unconditionally zapping lock classes into only zapping lock classes if
> > > debug_lock == true. Since the new behavior can cause cat /proc/lockdep to
> > > crash due to a NULL pointer dereference, restore the pre-v5.1 behavior.
> > 
> > Can you elaborate on this NULL dereference please, and why this patch fixes
> > it?
> 
> Not zapping lock classes if debug_lock == false leaves dangling pointers in
> several lockdep datastructures, e.g. lock_class::name in the all_lock_classes
> list. The shell command "cat /proc/lockdep" causes the kernel to iterate the
> all_lock_classes list. Hence the "unable to handle kernel paging request"
> issue that Shenghui encountered by running cat /proc/lockdep. Please let me
> know if you would like me to repost this patch with a more detailed
> description.

That would help me, at least (maybe with the crash log). Thanks.

> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Will Deacon <will.deacon@arm.com>
> > > Cc: Waiman Long <longman@redhat.com>
> > > Cc: shenghui <shhuiw@foxmail.com>
> > > Reported-by: shenghui <shhuiw@foxmail.com>
> > > Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use") # v5.1-rc1.
> > > Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> > > ---
> > >  kernel/locking/lockdep.c | 23 ++++++-----------------
> > >  1 file changed, 6 insertions(+), 17 deletions(-)
> > > 
> > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > > index 34cdcbedda49..70480e4f8f5d 100644
> > > --- a/kernel/locking/lockdep.c
> > > +++ b/kernel/locking/lockdep.c
> > > @@ -4689,8 +4689,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
> > >  		return;
> > >  
> > >  	raw_local_irq_save(flags);
> > > -	if (!graph_lock())
> > > -		goto out_irq;
> > > +	arch_spin_lock(&lockdep_lock);
> > 
> > This also throws out the recursion counting. Is that ok?
> 
> I think that that's OK. My understanding is that lockdep keeps track of
> recursion to avoid that lockdep_lock is locked recursively. However, none
> of the functions modified by this patch are called with that lock held.

Might be worth adding a comment to that effect, so people don't change that
in future.

Will

Re: [PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled

[Bart Van Assche]

On Wed, 2019-04-03 at 13:44 +0100, Will Deacon wrote:
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -4689,8 +4689,7 @@ static void free_zapped_rcu(struct rcu_head *ch)
> >  		return;
> >  
> >  	raw_local_irq_save(flags);
> > -	if (!graph_lock())
> > -		goto out_irq;
> > +	arch_spin_lock(&lockdep_lock);
> 
> This also throws out the recursion counting. Is that ok?

Apparently not, as the build bot just figured out. I will restore recursion
counting. See also
https://lore.kernel.org/lkml/5ca4ea66.eVtXtf5CmqRjP7xF%25lkp@intel.com/.

Bart.