[PATCH v4 01/27] lockdep: Teach lockdep about "USED" <- "IN-NMI" inversions

Peter Zijlstra
2020-02-21T13:34:17Z

nmi_enter() does lockdep_off() and hence lockdep ignores everything.

And NMI context makes it impossible to do full IN-NMI tracking like we
do IN-HARDIRQ, that could result in graph_lock recursion.

However, since look_up_lock_class() is lockless, we can find the class
of a lock that has prior use and detect IN-NMI after USED, just not
USED after IN-NMI.

NOTE: By shifting the lockdep_off() recursion count to bit-16, we can
easily differentiate between actual recursion and off.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/locking/lockdep.c | 53 ++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 50 insertions(+), 3 deletions(-)

--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -379,13 +379,13 @@ void lockdep_init_task(struct task_struc

void lockdep_off(void)
{
- current->lockdep_recursion++;
+ current->lockdep_recursion += BIT(16);
}
EXPORT_SYMBOL(lockdep_off);

void lockdep_on(void)
{
- current->lockdep_recursion--;
+ current->lockdep_recursion -= BIT(16);
}
EXPORT_SYMBOL(lockdep_on);

@@ -575,6 +575,7 @@ static const char *usage_str[] =
#include "lockdep_states.h"
#undef LOCKDEP_STATE
[LOCK_USED] = "INITIAL USE",
+ [LOCK_USAGE_STATES] = "IN-NMI",
};
#endif

@@ -787,6 +788,7 @@ static int count_matching_names(struct l
return count + 1;
}

+/* used from NMI context -- must be lockless */
static inline struct lock_class *
look_up_lock_class(const struct lockdep_map *lock, unsigned int subclass)
{
@@ -4463,6 +4465,34 @@ void lock_downgrade(struct lockdep_map *
}
EXPORT_SYMBOL_GPL(lock_downgrade);

+/* NMI context !!! */
+static void verify_lock_unused(struct lockdep_map *lock, struct held_lock *hlock, int subclass)
+{
+ struct lock_class *class = look_up_lock_class(lock, subclass);
+
+ /* if it doesn't have a class (yet), it certainly hasn't been used yet */
+ if (!class)
+ return;
+
+ if (!(class->usage_mask & LOCK_USED))
+ return;
+
+ hlock->class_idx = class - lock_classes;
+
+ print_usage_bug(current, hlock, LOCK_USED, LOCK_USAGE_STATES);
+}
+
+static bool lockdep_nmi(void)
+{
+ if (current->lockdep_recursion & 0xFFFF)
+ return false;
+
+ if (!in_nmi())
+ return false;
+
+ return true;
+}
+
/*
* We are not always called with irqs disabled - do that here,
* and also avoid lockdep recursion:
@@ -4473,8 +4503,25 @@ void lock_acquire(struct lockdep_map *lo
{
unsigned long flags;

- if (unlikely(current->lockdep_recursion))
+ if (unlikely(current->lockdep_recursion)) {
+ /* XXX allow trylock from NMI ?!? */
+ if (lockdep_nmi() && !trylock) {
+ struct held_lock hlock;
+
+ hlock.acquire_ip = ip;
+ hlock.instance = lock;
+ hlock.nest_lock = nest_lock;
+ hlock.irq_context = 2; // XXX
+ hlock.trylock = trylock;
+ hlock.read = read;
+ hlock.check = check;
+ hlock.hardirqs_off = true;
+ hlock.references = 0;
+
+ verify_lock_unused(lock, &hlock, subclass);
+ }
return;
+ }

raw_local_irq_save(flags);
check_flags(flags);