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From:   Boqun Feng <boqun.feng@gmail.com>
To:     linux-kernel@vger.kernel.org, rcu@vger.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Qian Cai <cai@lca.pw>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>
Subject: [PATCH] locking/lockdep: Avoid recursion in lockdep_count_{for,back}ward_deps()
Date:   Thu, 12 Mar 2020 23:12:55 +0800
Message-Id: <20200312151258.128036-1-boqun.feng@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Qian Cai reported a bug when PROVE_RCU_LIST=y, and read on /proc/lockdep
triggered a warning:

[26405.676199][ T3548] DEBUG_LOCKS_WARN_ON(current->hardirqs_enabled)
...
[26406.402408][ T3548] Call Trace:
[26406.416739][ T3548]  lock_is_held_type+0x5d/0x150
[26406.438262][ T3548]  ? rcu_lockdep_current_cpu_online+0x64/0x80
[26406.466463][ T3548]  rcu_read_lock_any_held+0xac/0x100
[26406.490105][ T3548]  ? rcu_read_lock_held+0xc0/0xc0
[26406.513258][ T3548]  ? __slab_free+0x421/0x540
[26406.535012][ T3548]  ? kasan_kmalloc+0x9/0x10
[26406.555901][ T3548]  ? __kmalloc_node+0x1d7/0x320
[26406.578668][ T3548]  ? kvmalloc_node+0x6f/0x80
[26406.599872][ T3548]  __bfs+0x28a/0x3c0
[26406.617075][ T3548]  ? class_equal+0x30/0x30
[26406.637524][ T3548]  lockdep_count_forward_deps+0x11a/0x1a0

The warning got triggered because lockdep_count_forward_deps() call
__bfs() without current->lockdep_recursion being set, as a result
a lockdep internal function (__bfs()) is checked by lockdep, which is
unexpected, and the inconsistency between the irq-off state and the
state traced by lockdep caused the warning.

Apart from this warning, lockdep internal functions like __bfs() should
always be protected by current->lockdep_recursion to avoid potential
deadlocks and data inconsistency, therefore add the
current->lockdep_recursion on-and-off section to protect __bfs() in both
lockdep_count_forward_deps() and lockdep_count_backward_deps()

Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 kernel/locking/lockdep.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 32406ef0d6a2..5142a6b11bf5 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1719,9 +1719,11 @@ unsigned long lockdep_count_forward_deps(struct lock_class *class)
 	this.class = class;
 
 	raw_local_irq_save(flags);
+	current->lockdep_recursion = 1;
 	arch_spin_lock(&lockdep_lock);
 	ret = __lockdep_count_forward_deps(&this);
 	arch_spin_unlock(&lockdep_lock);
+	current->lockdep_recursion = 0;
 	raw_local_irq_restore(flags);
 
 	return ret;
@@ -1746,9 +1748,11 @@ unsigned long lockdep_count_backward_deps(struct lock_class *class)
 	this.class = class;
 
 	raw_local_irq_save(flags);
+	current->lockdep_recursion = 1;
 	arch_spin_lock(&lockdep_lock);
 	ret = __lockdep_count_backward_deps(&this);
 	arch_spin_unlock(&lockdep_lock);
+	current->lockdep_recursion = 0;
 	raw_local_irq_restore(flags);
 
 	return ret;
-- 
2.25.1