[PATCH] bcache: fix deadlock in bcache_allocator()

[Andrea Righi <andrea.righi@canonical.com>]

bcache_allocator() can call the following:

 bch_allocator_thread()
  -> bch_prio_write()
     -> bch_bucket_alloc()
        -> wait on &ca->set->bucket_wait

But the wake up event on bucket_wait is supposed to come from
bch_allocator_thread() itself => deadlock:

 [ 242.888435] INFO: task bcache_allocato:9015 blocked for more than 120 seconds.
 [ 242.893786] Not tainted 4.20.0-042000rc3-generic #201811182231
 [ 242.896669] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 [ 242.900428] bcache_allocato D 0 9015 2 0x80000000
 [ 242.900434] Call Trace:
 [ 242.900448] __schedule+0x2a2/0x880
 [ 242.900455] ? __schedule+0x2aa/0x880
 [ 242.900462] schedule+0x2c/0x80
 [ 242.900480] bch_bucket_alloc+0x19d/0x380 [bcache]
 [ 242.900503] ? wait_woken+0x80/0x80
 [ 242.900519] bch_prio_write+0x190/0x340 [bcache]
 [ 242.900530] bch_allocator_thread+0x482/0xd10 [bcache]
 [ 242.900535] kthread+0x120/0x140
 [ 242.900546] ? bch_invalidate_one_bucket+0x80/0x80 [bcache]
 [ 242.900549] ? kthread_park+0x90/0x90
 [ 242.900554] ret_from_fork+0x35/0x40

Fix by making the call to bch_prio_write() non-blocking, so that
bch_allocator_thread() never waits on itself.

Moreover, make sure to wake up the garbage collector thread when
bch_prio_write() is failing to allocate buckets.

BugLink: https://bugs.launchpad.net/bugs/1784665
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
---
 drivers/md/bcache/alloc.c  |  6 +++++-
 drivers/md/bcache/bcache.h |  2 +-
 drivers/md/bcache/super.c  | 13 +++++++++----
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
index f8986effcb50..0797587600c7 100644
--- a/drivers/md/bcache/alloc.c
+++ b/drivers/md/bcache/alloc.c
@@ -377,7 +377,11 @@ static int bch_allocator_thread(void *arg)
 			if (!fifo_full(&ca->free_inc))
 				goto retry_invalidate;
 
-			bch_prio_write(ca);
+			if (bch_prio_write(ca, false) < 0) {
+				ca->invalidate_needs_gc = 1;
+				wake_up_gc(ca->set);
+				goto retry_invalidate;
+			}
 		}
 	}
 out:
diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index fdf75352e16a..dc5106b21260 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -979,7 +979,7 @@ bool bch_cached_dev_error(struct cached_dev *dc);
 __printf(2, 3)
 bool bch_cache_set_error(struct cache_set *c, const char *fmt, ...);
 
-void bch_prio_write(struct cache *ca);
+int bch_prio_write(struct cache *ca, bool wait);
 void bch_write_bdev_super(struct cached_dev *dc, struct closure *parent);
 
 extern struct workqueue_struct *bcache_wq;
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 1b63ac876169..6598b457df1a 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -525,7 +525,7 @@ static void prio_io(struct cache *ca, uint64_t bucket, int op,
 	closure_sync(cl);
 }
 
-void bch_prio_write(struct cache *ca)
+int bch_prio_write(struct cache *ca, bool wait)
 {
 	int i;
 	struct bucket *b;
@@ -560,8 +560,12 @@ void bch_prio_write(struct cache *ca)
 		p->magic	= pset_magic(&ca->sb);
 		p->csum		= bch_crc64(&p->magic, bucket_bytes(ca) - 8);
 
-		bucket = bch_bucket_alloc(ca, RESERVE_PRIO, true);
-		BUG_ON(bucket == -1);
+		bucket = bch_bucket_alloc(ca, RESERVE_PRIO, wait);
+		if (bucket == -1) {
+			if (!wait)
+				return -ENOMEM;
+			BUG_ON(1);
+		}
 
 		mutex_unlock(&ca->set->bucket_lock);
 		prio_io(ca, bucket, REQ_OP_WRITE, 0);
@@ -589,6 +593,7 @@ void bch_prio_write(struct cache *ca)
 
 		ca->prio_last_buckets[i] = ca->prio_buckets[i];
 	}
+	return 0;
 }
 
 static void prio_read(struct cache *ca, uint64_t bucket)
@@ -1903,7 +1908,7 @@ static int run_cache_set(struct cache_set *c)
 
 		mutex_lock(&c->bucket_lock);
 		for_each_cache(ca, c, i)
-			bch_prio_write(ca);
+			bch_prio_write(ca, true);
 		mutex_unlock(&c->bucket_lock);
 
 		err = "cannot allocate new UUID bucket";
-- 
2.20.1