diff options
author | Coly Li <colyli@suse.de> | 2019-06-28 14:59:58 +0300 |
---|---|---|
committer | Jens Axboe <axboe@kernel.dk> | 2019-06-28 16:39:18 +0300 |
commit | 50a260e859964002dab162513a10f91ae9d3bcd3 (patch) | |
tree | 827d1975dbd83780959c97d6e83f7770d1e87c16 | |
parent | d91ce7574daf48a4567ba62733d43284f5d2a3f4 (diff) | |
download | linux-50a260e859964002dab162513a10f91ae9d3bcd3.tar.xz |
bcache: fix race in btree_flush_write()
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@suse.de>
Reported-and-tested-by: kbuild test robot <lkp@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
-rw-r--r-- | drivers/md/bcache/btree.c | 28 | ||||
-rw-r--r-- | drivers/md/bcache/btree.h | 2 | ||||
-rw-r--r-- | drivers/md/bcache/journal.c | 7 |
3 files changed, 36 insertions, 1 deletions
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c index 846306c3a887..ba434d9ac720 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> /* @@ -659,12 +659,25 @@ static int mca_reap(struct btree *b, unsigned int min_order, bool flush) up(&b->io_mutex); } +retry: /* * 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. */ mutex_lock(&b->write_lock); + /* + * 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); mutex_unlock(&b->write_lock); @@ -1081,7 +1094,20 @@ static void btree_node_free(struct btree *b) BUG_ON(b == b->c->root); +retry: mutex_lock(&b->write_lock); + /* + * 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 d1c72ef64edf..76cfd121a486 100644 --- a/drivers/md/bcache/btree.h +++ b/drivers/md/bcache/btree.h @@ -158,11 +158,13 @@ enum btree_flags { BTREE_NODE_io_error, BTREE_NODE_dirty, BTREE_NODE_write_idx, + BTREE_NODE_journal_flush, }; BTREE_FLAG(io_error); BTREE_FLAG(dirty); BTREE_FLAG(write_idx); +BTREE_FLAG(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 1218e3cada3c..a1e3e1fcea6e 100644 --- a/drivers/md/bcache/journal.c +++ b/drivers/md/bcache/journal.c @@ -430,6 +430,7 @@ static void btree_flush_write(struct cache_set *c) retry: best = NULL; + mutex_lock(&c->bucket_lock); for_each_cached_btree(b, c, i) if (btree_current_write(b)->journal) { if (!best) @@ -442,15 +443,21 @@ retry: } b = best; + if (b) + set_btree_node_journal_flush(b); + mutex_unlock(&c->bucket_lock); + if (b) { mutex_lock(&b->write_lock); if (!btree_current_write(b)->journal) { + clear_bit(BTREE_NODE_journal_flush, &b->flags); mutex_unlock(&b->write_lock); /* We raced */ goto retry; } __bch_btree_node_write(b, NULL); + clear_bit(BTREE_NODE_journal_flush, &b->flags); mutex_unlock(&b->write_lock); } } |