Re: bcache-register hang after reboot

[Mauricio Faria de Oliveira <mfo@canonical.com>]

Hey Kent and Coly,

It turns out that, at least for the disk image that reproduces the issue,
the closure from bch_btree_set_root() to bch_journal_meta() doesn't make
a difference; the stall is in bch_journal() -> journal_wait_for_write().

So the previous suggestion to skip bch_journal_meta() altogether works,
to get things going.. of course, checking for journal replay/full case.

What do you think of this patch?

It simply checks the conditions in run_cache_set() for bch_journal_replay().
(it starts w/ unlikely(!CACHE_SET_RUNNING) to quickly get to the usual case,
and apparently has an extra strict check for !gc_thread, just in case. 
And it is journal_full() only, as the !journal_full() case in journal_wait_
for_write() seems to be handled via another function per the comment.)

This works w/ the disk image here.

Thanks!
Mauricio

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 72abe5cf4b12..bedeffc3ae28 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -2477,9 +2477,6 @@ int bch_btree_insert(struct cache_set *c, struct keylist *keys,
 void bch_btree_set_root(struct btree *b)
 {
 	unsigned int i;
-	struct closure cl;
-
-	closure_init_stack(&cl);
 
 	trace_bcache_btree_set_root(b);
 
@@ -2494,8 +2491,18 @@ void bch_btree_set_root(struct btree *b)
 
 	b->c->root = b;
 
-	bch_journal_meta(b->c, &cl);
-	closure_sync(&cl);
+	/* Don't journal during replay if journal is full (prevents deadlock) */
+	if (unlikely(!test_bit(CACHE_SET_RUNNING, &b->c->flags)) &&
+	    CACHE_SYNC(&b->c->cache->sb) && b->c->gc_thread == NULL &&
+	    journal_full(&b->c->journal)) {
+		pr_info("Not journaling new root (replay with full journal)\n");
+	} else {
+		struct closure cl;
+
+		closure_init_stack(&cl);
+		bch_journal_meta(b->c, &cl);
+		closure_sync(&cl);
+	}
 }
 
 /* Map across nodes or keys */