[PATCH AUTOSEL 5.5 404/542] debugobjects: Fix various data races

From: Sasha Levin
Date: Fri Feb 14 2020 - 12:55:45 EST

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From: Marco Elver <elver@xxxxxxxxxx>

[ Upstream commit 35fd7a637c42bb54ba4608f4d40ae6e55fc88781 ]

The counters obj_pool_free, and obj_nr_tofree, and the flag obj_freeing are
read locklessly outside the pool_lock critical sections. If read with plain
accesses, this would result in data races.

This is addressed as follows:

* reads outside critical sections become READ_ONCE()s (pairing with
WRITE_ONCE()s added);

* writes become WRITE_ONCE()s (pairing with READ_ONCE()s added); since
writes happen inside critical sections, only the write and not the read
of RMWs needs to be atomic, thus WRITE_ONCE(var, var +/- X) is
sufficient.

The data races were reported by KCSAN:

BUG: KCSAN: data-race in __free_object / fill_pool

write to 0xffffffff8beb04f8 of 4 bytes by interrupt on cpu 1:
__free_object+0x1ee/0x8e0 lib/debugobjects.c:404
__debug_check_no_obj_freed+0x199/0x330 lib/debugobjects.c:969
debug_check_no_obj_freed+0x3c/0x44 lib/debugobjects.c:994
slab_free_hook mm/slub.c:1422 [inline]

read to 0xffffffff8beb04f8 of 4 bytes by task 1 on cpu 2:
fill_pool+0x3d/0x520 lib/debugobjects.c:135
__debug_object_init+0x3c/0x810 lib/debugobjects.c:536
debug_object_init lib/debugobjects.c:591 [inline]
debug_object_activate+0x228/0x320 lib/debugobjects.c:677
debug_rcu_head_queue kernel/rcu/rcu.h:176 [inline]

BUG: KCSAN: data-race in __debug_object_init / fill_pool

read to 0xffffffff8beb04f8 of 4 bytes by task 10 on cpu 6:
fill_pool+0x3d/0x520 lib/debugobjects.c:135
__debug_object_init+0x3c/0x810 lib/debugobjects.c:536
debug_object_init_on_stack+0x39/0x50 lib/debugobjects.c:606
init_timer_on_stack_key kernel/time/timer.c:742 [inline]

write to 0xffffffff8beb04f8 of 4 bytes by task 1 on cpu 3:
alloc_object lib/debugobjects.c:258 [inline]
__debug_object_init+0x717/0x810 lib/debugobjects.c:544
debug_object_init lib/debugobjects.c:591 [inline]
debug_object_activate+0x228/0x320 lib/debugobjects.c:677
debug_rcu_head_queue kernel/rcu/rcu.h:176 [inline]

BUG: KCSAN: data-race in free_obj_work / free_object

read to 0xffffffff9140c190 of 4 bytes by task 10 on cpu 6:
free_object+0x4b/0xd0 lib/debugobjects.c:426
debug_object_free+0x190/0x210 lib/debugobjects.c:824
destroy_timer_on_stack kernel/time/timer.c:749 [inline]

write to 0xffffffff9140c190 of 4 bytes by task 93 on cpu 1:
free_obj_work+0x24f/0x480 lib/debugobjects.c:313
process_one_work+0x454/0x8d0 kernel/workqueue.c:2264
worker_thread+0x9a/0x780 kernel/workqueue.c:2410

Reported-by: Qian Cai <cai@xxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200116185529.11026-1-elver@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
lib/debugobjects.c | 46 +++++++++++++++++++++++++---------------------
1 file changed, 25 insertions(+), 21 deletions(-)

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 61261195f5b60..48054dbf1b51f 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -132,14 +132,18 @@ static void fill_pool(void)
struct debug_obj *obj;
unsigned long flags;

- if (likely(obj_pool_free >= debug_objects_pool_min_level))
+ if (likely(READ_ONCE(obj_pool_free) >= debug_objects_pool_min_level))
return;

/*
* Reuse objs from the global free list; they will be reinitialized
* when allocating.
+ *
+ * Both obj_nr_tofree and obj_pool_free are checked locklessly; the
+ * READ_ONCE()s pair with the WRITE_ONCE()s in pool_lock critical
+ * sections.
*/
- while (obj_nr_tofree && (obj_pool_free < obj_pool_min_free)) {
+ while (READ_ONCE(obj_nr_tofree) && (READ_ONCE(obj_pool_free) < obj_pool_min_free)) {
raw_spin_lock_irqsave(&pool_lock, flags);
/*
* Recheck with the lock held as the worker thread might have
@@ -148,9 +152,9 @@ static void fill_pool(void)
while (obj_nr_tofree && (obj_pool_free < obj_pool_min_free)) {
obj = hlist_entry(obj_to_free.first, typeof(*obj), node);
hlist_del(&obj->node);
- obj_nr_tofree--;
+ WRITE_ONCE(obj_nr_tofree, obj_nr_tofree - 1);
hlist_add_head(&obj->node, &obj_pool);
- obj_pool_free++;
+ WRITE_ONCE(obj_pool_free, obj_pool_free + 1);
}
raw_spin_unlock_irqrestore(&pool_lock, flags);
}
@@ -158,7 +162,7 @@ static void fill_pool(void)
if (unlikely(!obj_cache))
return;

- while (obj_pool_free < debug_objects_pool_min_level) {
+ while (READ_ONCE(obj_pool_free) < debug_objects_pool_min_level) {
struct debug_obj *new[ODEBUG_BATCH_SIZE];
int cnt;

@@ -174,7 +178,7 @@ static void fill_pool(void)
while (cnt) {
hlist_add_head(&new[--cnt]->node, &obj_pool);
debug_objects_allocated++;
- obj_pool_free++;
+ WRITE_ONCE(obj_pool_free, obj_pool_free + 1);
}
raw_spin_unlock_irqrestore(&pool_lock, flags);
}
@@ -236,7 +240,7 @@ alloc_object(void *addr, struct debug_bucket *b, struct debug_obj_descr *descr)
obj = __alloc_object(&obj_pool);
if (obj) {
obj_pool_used++;
- obj_pool_free--;
+ WRITE_ONCE(obj_pool_free, obj_pool_free - 1);

/*
* Looking ahead, allocate one batch of debug objects and
@@ -255,7 +259,7 @@ alloc_object(void *addr, struct debug_bucket *b, struct debug_obj_descr *descr)
&percpu_pool->free_objs);
percpu_pool->obj_free++;
obj_pool_used++;
- obj_pool_free--;
+ WRITE_ONCE(obj_pool_free, obj_pool_free - 1);
}
}

@@ -309,8 +313,8 @@ static void free_obj_work(struct work_struct *work)
obj = hlist_entry(obj_to_free.first, typeof(*obj), node);
hlist_del(&obj->node);
hlist_add_head(&obj->node, &obj_pool);
- obj_pool_free++;
- obj_nr_tofree--;
+ WRITE_ONCE(obj_pool_free, obj_pool_free + 1);
+ WRITE_ONCE(obj_nr_tofree, obj_nr_tofree - 1);
}
raw_spin_unlock_irqrestore(&pool_lock, flags);
return;
@@ -324,7 +328,7 @@ static void free_obj_work(struct work_struct *work)
if (obj_nr_tofree) {
hlist_move_list(&obj_to_free, &tofree);
debug_objects_freed += obj_nr_tofree;
- obj_nr_tofree = 0;
+ WRITE_ONCE(obj_nr_tofree, 0);
}
raw_spin_unlock_irqrestore(&pool_lock, flags);

@@ -375,10 +379,10 @@ static void __free_object(struct debug_obj *obj)
obj_pool_used--;

if (work) {
- obj_nr_tofree++;
+ WRITE_ONCE(obj_nr_tofree, obj_nr_tofree + 1);
hlist_add_head(&obj->node, &obj_to_free);
if (lookahead_count) {
- obj_nr_tofree += lookahead_count;
+ WRITE_ONCE(obj_nr_tofree, obj_nr_tofree + lookahead_count);
obj_pool_used -= lookahead_count;
while (lookahead_count) {
hlist_add_head(&objs[--lookahead_count]->node,
@@ -396,15 +400,15 @@ static void __free_object(struct debug_obj *obj)
for (i = 0; i < ODEBUG_BATCH_SIZE; i++) {
obj = __alloc_object(&obj_pool);
hlist_add_head(&obj->node, &obj_to_free);
- obj_pool_free--;
- obj_nr_tofree++;
+ WRITE_ONCE(obj_pool_free, obj_pool_free - 1);
+ WRITE_ONCE(obj_nr_tofree, obj_nr_tofree + 1);
}
}
} else {
- obj_pool_free++;
+ WRITE_ONCE(obj_pool_free, obj_pool_free + 1);
hlist_add_head(&obj->node, &obj_pool);
if (lookahead_count) {
- obj_pool_free += lookahead_count;
+ WRITE_ONCE(obj_pool_free, obj_pool_free + lookahead_count);
obj_pool_used -= lookahead_count;
while (lookahead_count) {
hlist_add_head(&objs[--lookahead_count]->node,
@@ -423,7 +427,7 @@ static void __free_object(struct debug_obj *obj)
static void free_object(struct debug_obj *obj)
{
__free_object(obj);
- if (!obj_freeing && obj_nr_tofree) {
+ if (!READ_ONCE(obj_freeing) && READ_ONCE(obj_nr_tofree)) {
WRITE_ONCE(obj_freeing, true);
schedule_delayed_work(&debug_obj_work, ODEBUG_FREE_WORK_DELAY);
}
@@ -982,7 +986,7 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size)
debug_objects_maxchecked = objs_checked;

/* Schedule work to actually kmem_cache_free() objects */
- if (!obj_freeing && obj_nr_tofree) {
+ if (!READ_ONCE(obj_freeing) && READ_ONCE(obj_nr_tofree)) {
WRITE_ONCE(obj_freeing, true);
schedule_delayed_work(&debug_obj_work, ODEBUG_FREE_WORK_DELAY);
}
@@ -1008,12 +1012,12 @@ static int debug_stats_show(struct seq_file *m, void *v)
seq_printf(m, "max_checked :%d\n", debug_objects_maxchecked);
seq_printf(m, "warnings :%d\n", debug_objects_warnings);
seq_printf(m, "fixups :%d\n", debug_objects_fixups);
- seq_printf(m, "pool_free :%d\n", obj_pool_free + obj_percpu_free);
+ seq_printf(m, "pool_free :%d\n", READ_ONCE(obj_pool_free) + obj_percpu_free);
seq_printf(m, "pool_pcp_free :%d\n", obj_percpu_free);
seq_printf(m, "pool_min_free :%d\n", obj_pool_min_free);
seq_printf(m, "pool_used :%d\n", obj_pool_used - obj_percpu_free);
seq_printf(m, "pool_max_used :%d\n", obj_pool_max_used);
- seq_printf(m, "on_free_list :%d\n", obj_nr_tofree);
+ seq_printf(m, "on_free_list :%d\n", READ_ONCE(obj_nr_tofree));
seq_printf(m, "objs_allocated:%d\n", debug_objects_allocated);
seq_printf(m, "objs_freed :%d\n", debug_objects_freed);
return 0;
--
2.20.1

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