[PATCH 5.2 20/37] bcache: fix race in btree_flush_write()

From: Greg Kroah-Hartman
Date: Fri Sep 13 2019 - 09:21:21 EST

There is a race between mca_reap(), btree_node_free() and journal code
btree_flush_write(), which results very rare and strange deadlock or
panic and are very hard to reproduce.

Let me explain how the race happens. In btree_flush_write() one btree
node with oldest journal pin is selected, then it is flushed to cache
device, the select-and-flush is a two steps operation. Between these two
steps, there are something may happen inside the race window,
- The selected btree node was reaped by mca_reap() and allocated to
other requesters for other btree node.
- The slected btree node was selected, flushed and released by mca
shrink callback bch_mca_scan().
When btree_flush_write() tries to flush the selected btree node, firstly
b->write_lock is held by mutex_lock(). If the race happens and the
memory of selected btree node is allocated to other btree node, if that
btree node's write_lock is held already, a deadlock very probably
happens here. A worse case is the memory of the selected btree node is
released, then all references to this btree node (e.g. b->write_lock)
will trigger NULL pointer deference panic.

This race was introduced in commit cafe56359144 ("bcache: A block layer
cache"), and enlarged by commit c4dc2497d50d ("bcache: fix high CPU
occupancy during journal"), which selected 128 btree nodes and flushed
them one-by-one in a quite long time period.

Such race is not easy to reproduce before. On a Lenovo SR650 server with
48 Xeon cores, and configure 1 NVMe SSD as cache device, a MD raid0
device assembled by 3 NVMe SSDs as backing device, this race can be
observed around every 10,000 times btree_flush_write() gets called. Both
deadlock and kernel panic all happened as aftermath of the race.

The idea of the fix is to add a btree flag BTREE_NODE_journal_flush. It
is set when selecting btree nodes, and cleared after btree nodes
flushed. Then when mca_reap() selects a btree node with this bit set,
this btree node will be skipped. Since mca_reap() only reaps btree node
without BTREE_NODE_journal_flush flag, such race is avoided.

Once corner case should be noticed, that is btree_node_free(). It might
be called in some error handling code path. For example the following
code piece from btree_split(),
2149 err_free2:
2150 bkey_put(b->c, &n2->key);
2151 btree_node_free(n2);
2152 rw_unlock(true, n2);
2153 err_free1:
2154 bkey_put(b->c, &n1->key);
2155 btree_node_free(n1);
2156 rw_unlock(true, n1);
At line 2151 and 2155, the btree node n2 and n1 are released without
mac_reap(), so BTREE_NODE_journal_flush also needs to be checked here.
If btree_node_free() is called directly in such error handling path,
and the selected btree node has BTREE_NODE_journal_flush bit set, just
delay for 1 us and retry again. In this case this btree node won't
be skipped, just retry until the BTREE_NODE_journal_flush bit cleared,
and free the btree node memory.

Fixes: cafe56359144 ("bcache: A block layer cache")
Signed-off-by: Coly Li <colyli@xxxxxxx>
Reported-and-tested-by: kbuild test robot <lkp@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
drivers/md/bcache/btree.c | 28 +++++++++++++++++++++++++++-
drivers/md/bcache/btree.h | 2 ++
drivers/md/bcache/journal.c | 7 +++++++
3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 9788b2ee6638f..5cf3247e8afb2 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -35,7 +35,7 @@
#include <linux/rcupdate.h>
#include <linux/sched/clock.h>
#include <linux/rculist.h>
+#include <linux/delay.h>
#include <trace/events/bcache.h>

@@ -655,12 +655,25 @@ static int mca_reap(struct btree *b, unsigned int min_order, bool flush)

* BTREE_NODE_dirty might be cleared in btree_flush_btree() by
* __bch_btree_node_write(). To avoid an extra flush, acquire
* b->write_lock before checking BTREE_NODE_dirty bit.
+ /*
+ * If this btree node is selected in btree_flush_write() by journal
+ * code, delay and retry until the node is flushed by journal code
+ * and BTREE_NODE_journal_flush bit cleared by btree_flush_write().
+ */
+ if (btree_node_journal_flush(b)) {
+ pr_debug("bnode %p is flushing by journal, retry", b);
+ mutex_unlock(&b->write_lock);
+ udelay(1);
+ goto retry;
+ }
if (btree_node_dirty(b))
__bch_btree_node_write(b, &cl);
@@ -1077,7 +1090,20 @@ static void btree_node_free(struct btree *b)

BUG_ON(b == b->c->root);

+ /*
+ * If the btree node is selected and flushing in btree_flush_write(),
+ * delay and retry until the BTREE_NODE_journal_flush bit cleared,
+ * then it is safe to free the btree node here. Otherwise this btree
+ * node will be in race condition.
+ */
+ if (btree_node_journal_flush(b)) {
+ mutex_unlock(&b->write_lock);
+ pr_debug("bnode %p journal_flush set, retry", b);
+ udelay(1);
+ goto retry;
+ }

if (btree_node_dirty(b)) {
btree_complete_write(b, btree_current_write(b));
diff --git a/drivers/md/bcache/btree.h b/drivers/md/bcache/btree.h
index d1c72ef64edf5..76cfd121a4861 100644
--- a/drivers/md/bcache/btree.h
+++ b/drivers/md/bcache/btree.h
@@ -158,11 +158,13 @@ enum btree_flags {
+ BTREE_NODE_journal_flush,


static inline struct btree_write *btree_current_write(struct btree *b)
diff --git a/drivers/md/bcache/journal.c b/drivers/md/bcache/journal.c
index cae2aff5e27ae..33556acdcf9cd 100644
--- a/drivers/md/bcache/journal.c
+++ b/drivers/md/bcache/journal.c
@@ -405,6 +405,7 @@ static void btree_flush_write(struct cache_set *c)
best = NULL;

+ mutex_lock(&c->bucket_lock);
for_each_cached_btree(b, c, i)
if (btree_current_write(b)->journal) {
if (!best)
@@ -417,9 +418,14 @@ retry:

b = best;
+ if (b)
+ set_btree_node_journal_flush(b);
+ mutex_unlock(&c->bucket_lock);
if (b) {
if (!btree_current_write(b)->journal) {
+ clear_bit(BTREE_NODE_journal_flush, &b->flags);
/* We raced */
@@ -427,6 +433,7 @@ retry:

__bch_btree_node_write(b, NULL);
+ clear_bit(BTREE_NODE_journal_flush, &b->flags);