We had a problem with io hung because it was waiting for c->root to
release the lock.
crash> cache_set.root -l cache_set.list ffffa03fde4c0050
root = 0xffff802ef454c800
crash> btree -o 0xffff802ef454c800 | grep rw_semaphore
[ffff802ef454c858] struct rw_semaphore lock;
crash> struct rw_semaphore ffff802ef454c858
struct rw_semaphore {
count = {
counter = -4294967297
},
wait_list = {
next = 0xffff00006786fc28,
prev = 0xffff00005d0efac8
},
wait_lock = {
raw_lock = {
{
val = {
counter = 0
},
{
locked = 0 '\000',
pending = 0 '\000'
},
{
locked_pending = 0,
tail = 0
}
}
}
},
osq = {
tail = {
counter = 0
}
},
owner = 0xffffa03fdc586603
}
The "counter = -4294967297" means that lock count is -1 and a write lock
is being attempted. Then, we found that there is a btree with a counter
of 1 in btree_cache_freeable.
crash> cache_set -l cache_set.list ffffa03fde4c0050 -o|grep btree_cache
[ffffa03fde4c1140] struct list_head btree_cache;
[ffffa03fde4c1150] struct list_head btree_cache_freeable;
[ffffa03fde4c1160] struct list_head btree_cache_freed;
[ffffa03fde4c1170] unsigned int btree_cache_used;
[ffffa03fde4c1178] wait_queue_head_t btree_cache_wait;
[ffffa03fde4c1190] struct task_struct *btree_cache_alloc_lock;
crash> list -H ffffa03fde4c1140|wc -l
973
crash> list -H ffffa03fde4c1150|wc -l
1123
crash> cache_set.btree_cache_used -l cache_set.list ffffa03fde4c0050
btree_cache_used = 2097
crash> list -s btree -l btree.list -H ffffa03fde4c1140|grep -E -A2 "^ lock = {" > btree_cache.txt
crash> list -s btree -l btree.list -H ffffa03fde4c1150|grep -E -A2 "^ lock = {" > btree_cache_freeable.txt
[root@node-3 127.0.0.1-2023-08-04-16:40:28]# pwd
/var/crash/127.0.0.1-2023-08-04-16:40:28
[root@node-3 127.0.0.1-2023-08-04-16:40:28]# cat btree_cache.txt|grep counter|grep -v "counter = 0"
[root@node-3 127.0.0.1-2023-08-04-16:40:28]# cat btree_cache_freeable.txt|grep counter|grep -v "counter = 0"
counter = 1
We found that this is a bug in bch_sectors_dirty_init() when locking c->root:
(1). Thread X has locked c->root(A) write.
(2). Thread Y failed to lock c->root(A), waiting for the lock(c->root A).
(3). Thread X bch_btree_set_root() changes c->root from A to B.
(4). Thread X releases the lock(c->root A).
(5). Thread Y successfully locks c->root(A).
(6). Thread Y releases the lock(c->root B).
down_write locked ---(1)----------------------┐
| |
| down_read waiting ---(2)----┐ |
| | ┌-------------┐ ┌-------------┐
bch_btree_set_root ===(3)========>> | c->root A | | c->root B |
| | └-------------┘ └-------------┘
up_write ---(4)---------------------┘ | |
| | |
down_read locked ---(5)-----------┘ |
| |
up_read ---(6)-----------------------------┘
Since c->root may change, the correct steps to lock c->root should be
the same as bch_root_usage(), compare after locking.
static unsigned int bch_root_usage(struct cache_set *c)
{
unsigned int bytes = 0;
struct bkey *k;
struct btree *b;
struct btree_iter iter;
goto lock_root;
do {
rw_unlock(false, b);
lock_root:
b = c->root;
rw_lock(false, b, b->level);
} while (b != c->root);
for_each_key_filter(&b->keys, k, &iter, bch_ptr_bad)
bytes += bkey_bytes(k);
rw_unlock(false, b);
return (bytes * 100) / btree_bytes(c);
}
Fixes: b144e45fc576 ("bcache: make bch_sectors_dirty_init() to be multithreaded")
Signed-off-by: Mingzhe Zou <mingzhe.zou@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
drivers/md/bcache/writeback.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 24c049067f61..bac916ba08c8 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -977,14 +977,22 @@ static int bch_btre_dirty_init_thread_nr(void)
void bch_sectors_dirty_init(struct bcache_device *d)
{
int i;
+ struct btree *b = NULL;
struct bkey *k = NULL;
struct btree_iter iter;
struct sectors_dirty_init op;
struct cache_set *c = d->c;
struct bch_dirty_init_state state;
+retry_lock:
+ b = c->root;
+ rw_lock(0, b, b->level);
+ if (b != c->root) {
+ rw_unlock(0, b);
+ goto retry_lock;
+ }
+
/* Just count root keys if no leaf node */
- rw_lock(0, c->root, c->root->level);
if (c->root->level == 0) {
bch_btree_op_init(&op.op, -1);
op.inode = d->id;
@@ -994,7 +1002,7 @@ void bch_sectors_dirty_init(struct bcache_device *d)
k, &iter, bch_ptr_invalid)
sectors_dirty_init_fn(&op.op, c->root, k);
- rw_unlock(0, c->root);
+ rw_unlock(0, b);
return;
}
@@ -1030,7 +1038,7 @@ void bch_sectors_dirty_init(struct bcache_device *d)
out:
/* Must wait for all threads to stop. */
wait_event(state.wait, atomic_read(&state.started) == 0);
- rw_unlock(0, c->root);
+ rw_unlock(0, b);
}
void bch_cached_dev_writeback_init(struct cached_dev *dc)
--
2.17.1.windows.2
