[PATCH v2] 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. 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. Since the new behavior can cause cat /proc/lockdep to
crash, restore the pre-v5.1 behavior.

This patch avoids that cat /proc/lockdep triggers the following crash
with debug_lock == false:

BUG: unable to handle kernel paging request at fffffbfff40ca448
PGD 13bfde067 P4D 13bfde067 PUD 13bf7a067 PMD 1167d3067 PTE 0
Oops: 0000 [#1] PREEMPT SMP KASAN
CPU: 4 PID: 4529 Comm: cat Tainted: G    B   W  O      5.1.0-rc1-dbg+ #4
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
RIP: 0010:__asan_load1+0x28/0x50
Call Trace:
 string+0xac/0x180
 vsnprintf+0x23e/0x820
 seq_vprintf+0x82/0xc0
 seq_printf+0x92/0xb0
 print_name+0x34/0xb0
 l_show+0x184/0x200
 seq_read+0x59e/0x6c0
 proc_reg_read+0x11f/0x170
 __vfs_read+0x4d/0x90
 vfs_read+0xc5/0x1f0
 ksys_read+0xab/0x130
 __x64_sys_read+0x43/0x50
 do_syscall_64+0x71/0x210
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: shenghui <shhuiw@xxxxxxxxxxx>
Reported-by: shenghui <shhuiw@xxxxxxxxxxx>
Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use") # v5.1-rc1.
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
Changes compared to v1:
- Restored lockdep recursion protection.
- Made patch description more detailed.

 kernel/locking/lockdep.c | 29 ++++++++++++-----------------
 1 file changed, 12 insertions(+), 17 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 34cdcbedda49..e16766ff184b 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4689,8 +4689,8 @@ 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);
+	current->lockdep_recursion = 1;
 
 	/* closed head */
 	pf = delayed_free.pf + (delayed_free.index ^ 1);
@@ -4702,8 +4702,8 @@ static void free_zapped_rcu(struct rcu_head *ch)
 	 */
 	call_rcu_zapped(delayed_free.pf + delayed_free.index);
 
-	graph_unlock();
-out_irq:
+	current->lockdep_recursion = 0;
+	arch_spin_unlock(&lockdep_lock);
 	raw_local_irq_restore(flags);
 }
 
@@ -4744,21 +4744,17 @@ 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);
+	current->lockdep_recursion = 1;
 	pf = get_pending_free();
 	__lockdep_free_key_range(pf, start, size);
 	call_rcu_zapped(pf);
-
-	graph_unlock();
-out_irq:
+	current->lockdep_recursion = 0;
+	arch_spin_unlock(&lockdep_lock);
 	raw_local_irq_restore(flags);
 
 	/*
@@ -4911,9 +4907,8 @@ 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);
+	current->lockdep_recursion = 1;
 	pf = get_pending_free();
 	hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
 		if (k == key) {
@@ -4925,8 +4920,8 @@ 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:
+	current->lockdep_recursion = 0;
+	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