locking/lockdep: Zap lock classes even with lock debugging disabled

The following commit:

  a0b0fd53e1 ("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" cash
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
  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

Reported-by: shenghui <shhuiw@foxmail.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Fixes: a0b0fd53e1 ("locking/lockdep: Free lock classes that are no longer in use") # v5.1-rc1.
Link: https://lkml.kernel.org/r/20190403233552.124673-1-bvanassche@acm.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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. */