lkml.org 
[lkml]   [2019]   [Mar]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH v3 09/18] locking/lockdep: Change the range of class_idx in held_lock struct
Date
held_lock->class_idx is used to point to the class of the held lock. The
index is shifted by 1 to make index 0 mean no class, which results in class
index shifting back and forth but is not worth doing so.

The reason is: (1) there will be no "no-class" held_lock to begin with, and
(2) index 0 seems to be used for error checking, but if something wrong
indeed happended, the index can't be counted on to distinguish it as that
something won't set the class_idx to 0 on purpose to tell us it is wrong.

Therefore, change the index to start from 0. This saves a lot of
back-and-forth shifts and save a slot back to lock_classes.

Since index 0 is now used for lock class, we change the initial chain key to
-1 to avoid key collision, which is due to the fact that __jhash_mix(0, 0, 0) = 0.
Actually, the initial chain key can be any arbitrary value other than 0.

In addition, we maintain a bitmap to keep track of the used lock classes,
and we check the validity of the held lock against that bitmap.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
include/linux/lockdep.h | 14 ++++++------
kernel/locking/lockdep.c | 59 ++++++++++++++++++++++++++++++++----------------
2 files changed, 46 insertions(+), 27 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 64d4565..0859d80 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -218,13 +218,8 @@ struct lock_chain {
};

#define MAX_LOCKDEP_KEYS_BITS 13
-/*
- * Subtract one because we offset hlock->class_idx by 1 in order
- * to make 0 mean no class. This avoids overflowing the class_idx
- * bitfield and hitting the BUG in hlock_class().
- */
-#define MAX_LOCKDEP_KEYS ((1UL << MAX_LOCKDEP_KEYS_BITS) - 1)
-#define INITIAL_CHAIN_KEY 0
+#define MAX_LOCKDEP_KEYS (1UL << MAX_LOCKDEP_KEYS_BITS)
+#define INITIAL_CHAIN_KEY -1

struct held_lock {
/*
@@ -249,6 +244,11 @@ struct held_lock {
u64 waittime_stamp;
u64 holdtime_stamp;
#endif
+ /*
+ * class_idx is zero-indexed; it points to the element in
+ * lock_classes this held lock instance belongs to. class_idx is in
+ * the range from 0 to (MAX_LOCKDEP_KEYS-1) inclusive.
+ */
unsigned int class_idx:MAX_LOCKDEP_KEYS_BITS;
/*
* The lock-stack is unified in that the lock chains of interrupt
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index c6363f7..f16d2f5 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -150,17 +150,28 @@ static inline int debug_locks_off_graph_unlock(void)
static
#endif
struct lock_class lock_classes[MAX_LOCKDEP_KEYS];
+static DECLARE_BITMAP(lock_classes_in_use, MAX_LOCKDEP_KEYS);

static inline struct lock_class *hlock_class(struct held_lock *hlock)
{
- if (!hlock->class_idx) {
+ unsigned int class_idx = hlock->class_idx;
+
+ /* Don't re-read hlock->class_idx, can't use READ_ONCE() on bitfield */
+ barrier();
+
+ if (!test_bit(class_idx, lock_classes_in_use)) {
/*
* Someone passed in garbage, we give up.
*/
DEBUG_LOCKS_WARN_ON(1);
return NULL;
}
- return lock_classes + hlock->class_idx - 1;
+
+ /*
+ * At this point, if the passed hlock->class_idx is still garbage,
+ * we just have to live with it
+ */
+ return lock_classes + class_idx;
}

#ifdef CONFIG_LOCK_STAT
@@ -604,19 +615,22 @@ static void print_lock(struct held_lock *hlock)
/*
* We can be called locklessly through debug_show_all_locks() so be
* extra careful, the hlock might have been released and cleared.
+ *
+ * If this indeed happens, lets pretend it does not hurt to continue
+ * to print the lock unless the hlock class_idx does not point to a
+ * registered class. The rationale here is: since we don't attempt
+ * to distinguish whether we are in this situation, if it just
+ * happened we can't count on class_idx to tell either.
*/
- unsigned int class_idx = hlock->class_idx;
+ struct lock_class *lock = hlock_class(hlock);

- /* Don't re-read hlock->class_idx, can't use READ_ONCE() on bitfields: */
- barrier();
-
- if (!class_idx || (class_idx - 1) >= MAX_LOCKDEP_KEYS) {
+ if (!lock) {
printk(KERN_CONT "<RELEASED>\n");
return;
}

printk(KERN_CONT "%p", hlock->instance);
- print_lock_name(lock_classes + class_idx - 1);
+ print_lock_name(lock);
printk(KERN_CONT ", at: %pS\n", (void *)hlock->acquire_ip);
}

@@ -871,7 +885,7 @@ static bool check_lock_chain_key(struct lock_chain *chain)
int i;

for (i = chain->base; i < chain->base + chain->depth; i++)
- chain_key = iterate_chain_key(chain_key, chain_hlocks[i] + 1);
+ chain_key = iterate_chain_key(chain_key, chain_hlocks[i]);
/*
* The 'unsigned long long' casts avoid that a compiler warning
* is reported when building tools/lib/lockdep.
@@ -1146,6 +1160,7 @@ static bool is_dynamic_key(const struct lock_class_key *key)
return NULL;
}
nr_lock_classes++;
+ __set_bit(class - lock_classes, lock_classes_in_use);
debug_atomic_inc(nr_unused_locks);
class->key = key;
class->name = lock->name;
@@ -2459,7 +2474,7 @@ static void print_chain_keys_chain(struct lock_chain *chain)
printk("depth: %u\n", chain->depth);
for (i = 0; i < chain->depth; i++) {
class_id = chain_hlocks[chain->base + i];
- chain_key = print_chain_key_iteration(class_id + 1, chain_key);
+ chain_key = print_chain_key_iteration(class_id, chain_key);

print_lock_name(lock_classes + class_id);
printk("\n");
@@ -2510,7 +2525,7 @@ static int check_no_collision(struct task_struct *curr,
}

for (j = 0; j < chain->depth - 1; j++, i++) {
- id = curr->held_locks[i].class_idx - 1;
+ id = curr->held_locks[i].class_idx;

if (DEBUG_LOCKS_WARN_ON(chain_hlocks[chain->base + j] != id)) {
print_collision(curr, hlock, chain);
@@ -2593,7 +2608,7 @@ static inline int add_chain_cache(struct task_struct *curr,
if (likely(nr_chain_hlocks + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
chain->base = nr_chain_hlocks;
for (j = 0; j < chain->depth - 1; j++, i++) {
- int lock_id = curr->held_locks[i].class_idx - 1;
+ int lock_id = curr->held_locks[i].class_idx;
chain_hlocks[chain->base + j] = lock_id;
}
chain_hlocks[chain->base + j] = class - lock_classes;
@@ -2773,10 +2788,12 @@ static void check_chain_key(struct task_struct *curr)
(unsigned long long)hlock->prev_chain_key);
return;
}
+
/*
- * Whoops ran out of static storage again?
+ * hlock->class_idx can't go beyond MAX_LOCKDEP_KEYS, but is
+ * it registered lock class index?
*/
- if (DEBUG_LOCKS_WARN_ON(hlock->class_idx > MAX_LOCKDEP_KEYS))
+ if (DEBUG_LOCKS_WARN_ON(!test_bit(hlock->class_idx, lock_classes_in_use)))
return;

if (prev_hlock && (prev_hlock->irq_context !=
@@ -3622,7 +3639,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
if (DEBUG_LOCKS_WARN_ON(depth >= MAX_LOCK_DEPTH))
return 0;

- class_idx = class - lock_classes + 1;
+ class_idx = class - lock_classes;

if (depth) {
hlock = curr->held_locks + depth - 1;
@@ -3684,9 +3701,9 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
* the hash, not class->key.
*/
/*
- * Whoops, we did it again.. ran straight out of our static allocation.
+ * Whoops, we did it again.. class_idx is invalid.
*/
- if (DEBUG_LOCKS_WARN_ON(class_idx > MAX_LOCKDEP_KEYS))
+ if (DEBUG_LOCKS_WARN_ON(!test_bit(class_idx, lock_classes_in_use)))
return 0;

chain_key = curr->curr_chain_key;
@@ -3801,7 +3818,7 @@ static int match_held_lock(const struct held_lock *hlock,
if (DEBUG_LOCKS_WARN_ON(!hlock->nest_lock))
return 0;

- if (hlock->class_idx == class - lock_classes + 1)
+ if (hlock->class_idx == class - lock_classes)
return 1;
}

@@ -3895,7 +3912,7 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,

lockdep_init_map(lock, name, key, 0);
class = register_lock_class(lock, subclass, 0);
- hlock->class_idx = class - lock_classes + 1;
+ hlock->class_idx = class - lock_classes;

curr->lockdep_depth = i;
curr->curr_chain_key = hlock->prev_chain_key;
@@ -4545,7 +4562,7 @@ static void remove_class_from_lock_chain(struct pending_free *pf,
recalc:
chain_key = INITIAL_CHAIN_KEY;
for (i = chain->base; i < chain->base + chain->depth; i++)
- chain_key = iterate_chain_key(chain_key, chain_hlocks[i] + 1);
+ chain_key = iterate_chain_key(chain_key, chain_hlocks[i]);
if (chain->depth && chain->chain_key == chain_key)
return;
/* Overwrite the chain key for concurrent RCU readers. */
@@ -4619,6 +4636,7 @@ static void zap_class(struct pending_free *pf, struct lock_class *class)
WRITE_ONCE(class->key, NULL);
WRITE_ONCE(class->name, NULL);
nr_lock_classes--;
+ __clear_bit(class - lock_classes, lock_classes_in_use);
} else {
WARN_ONCE(true, "%s() failed for class %s\n", __func__,
class->name);
@@ -4968,6 +4986,7 @@ void __init lockdep_init(void)

printk(" memory used by lock dependency info: %zu kB\n",
(sizeof(lock_classes) +
+ sizeof(lock_classes_in_use) +
sizeof(classhash_table) +
sizeof(list_entries) +
sizeof(list_entries_in_use) +
--
1.8.3.1
\
 
 \ /
  Last update: 2019-03-21 08:59    [W:0.082 / U:4.364 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site