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Subject[PATCH 5.8 079/118] locking/lockdep: Fix "USED" <- "IN-NMI" inversions
Date
From: peterz@infradead.org <peterz@infradead.org>

[ Upstream commit 23870f1227680d2aacff6f79c3ab2222bd04e86e ]

During the LPC RCU BoF Paul asked how come the "USED" <- "IN-NMI"
detector doesn't trip over rcu_read_lock()'s lockdep annotation.

Looking into this I found a very embarrasing typo in
verify_lock_unused():

- if (!(class->usage_mask & LOCK_USED))
+ if (!(class->usage_mask & LOCKF_USED))

fixing that will indeed cause rcu_read_lock() to insta-splat :/

The above typo means that instead of testing for: 0x100 (1 <<
LOCK_USED), we test for 8 (LOCK_USED), which corresponds to (1 <<
LOCK_ENABLED_HARDIRQ).

So instead of testing for _any_ used lock, it will only match any lock
used with interrupts enabled.

The rcu_read_lock() annotation uses .check=0, which means it will not
set any of the interrupt bits and will thus never match.

In order to properly fix the situation and allow rcu_read_lock() to
correctly work, split LOCK_USED into LOCK_USED and LOCK_USED_READ and by
having .read users set USED_READ and test USED, pure read-recursive
locks are permitted.

Fixes: f6f48e180404 ("lockdep: Teach lockdep about "USED" <- "IN-NMI" inversions")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20200902160323.GK1362448@hirez.programming.kicks-ass.net
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/locking/lockdep.c | 35 +++++++++++++++++++++++++-----
kernel/locking/lockdep_internals.h | 2 ++
2 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 29a8de4c50b90..a611dedac7d60 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3923,13 +3923,18 @@ static int separate_irq_context(struct task_struct *curr,
static int mark_lock(struct task_struct *curr, struct held_lock *this,
enum lock_usage_bit new_bit)
{
- unsigned int new_mask = 1 << new_bit, ret = 1;
+ unsigned int old_mask, new_mask, ret = 1;

if (new_bit >= LOCK_USAGE_STATES) {
DEBUG_LOCKS_WARN_ON(1);
return 0;
}

+ if (new_bit == LOCK_USED && this->read)
+ new_bit = LOCK_USED_READ;
+
+ new_mask = 1 << new_bit;
+
/*
* If already set then do not dirty the cacheline,
* nor do any checks:
@@ -3942,13 +3947,22 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
/*
* Make sure we didn't race:
*/
- if (unlikely(hlock_class(this)->usage_mask & new_mask)) {
- graph_unlock();
- return 1;
- }
+ if (unlikely(hlock_class(this)->usage_mask & new_mask))
+ goto unlock;

+ old_mask = hlock_class(this)->usage_mask;
hlock_class(this)->usage_mask |= new_mask;

+ /*
+ * Save one usage_traces[] entry and map both LOCK_USED and
+ * LOCK_USED_READ onto the same entry.
+ */
+ if (new_bit == LOCK_USED || new_bit == LOCK_USED_READ) {
+ if (old_mask & (LOCKF_USED | LOCKF_USED_READ))
+ goto unlock;
+ new_bit = LOCK_USED;
+ }
+
if (!(hlock_class(this)->usage_traces[new_bit] = save_trace()))
return 0;

@@ -3962,6 +3976,7 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
return 0;
}

+unlock:
graph_unlock();

/*
@@ -4896,12 +4911,20 @@ static void verify_lock_unused(struct lockdep_map *lock, struct held_lock *hlock
{
#ifdef CONFIG_PROVE_LOCKING
struct lock_class *class = look_up_lock_class(lock, subclass);
+ unsigned long mask = LOCKF_USED;

/* if it doesn't have a class (yet), it certainly hasn't been used yet */
if (!class)
return;

- if (!(class->usage_mask & LOCK_USED))
+ /*
+ * READ locks only conflict with USED, such that if we only ever use
+ * READ locks, there is no deadlock possible -- RCU.
+ */
+ if (!hlock->read)
+ mask |= LOCKF_USED_READ;
+
+ if (!(class->usage_mask & mask))
return;

hlock->class_idx = class - lock_classes;
diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
index baca699b94e91..b0be1560ed17a 100644
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -19,6 +19,7 @@ enum lock_usage_bit {
#include "lockdep_states.h"
#undef LOCKDEP_STATE
LOCK_USED,
+ LOCK_USED_READ,
LOCK_USAGE_STATES
};

@@ -40,6 +41,7 @@ enum {
#include "lockdep_states.h"
#undef LOCKDEP_STATE
__LOCKF(USED)
+ __LOCKF(USED_READ)
};

#define LOCKDEP_STATE(__STATE) LOCKF_ENABLED_##__STATE |
--
2.25.1


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