Btrfs: fix deadlock between alloc_mutex/chunk_mutex

This fixes a deadlock that happens between the alloc_mutex and chunk_mutex.
Process A comes in, decides to do a do_chunk_alloc, which takes the
chunk_mutex, and is holding the alloc_mutex because the only way you get to
do_chunk_alloc is by holding the alloc_mutex.  btrfs_alloc_chunk does its thing
and goes to insert a new item, which results in a cow of the block.

We get into del_pending_extents from there, where if we need to be rescheduled
we drop the alloc_mutex and schedule.  At this point process B comes in to do
an allocation and gets the alloc_mutex, and because process A did not do the
chunk allocation completely it thinks its a good time to do a chunk allocation
as well, and hangs on the chunk_mutex.

Process A wakes up and tries to take the alloc_mutex and cannot.  The way to
fix this is do a mutex_trylock() on chunk_mutex.  If we return 0 we didn't get
the lock, and if this is just a "hey it may be a good time to allocate a chunk"
then we just exit.  If we are trying to force an allocation then we reschedule
and keep trying to acquire the chunk_mutex.  If once we acquire it the space is
already full then we can just exit, otherwise we can continue with the chunk
allocation.  Thank you,

Signed-off-by: Josef Bacik <jbacik@redhat.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 677d5e774fad..db37b867e4f1 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -1505,7 +1505,7 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans,
 	u64 thresh;
 	u64 start;
 	u64 num_bytes;
-	int ret = 0;
+	int ret = 0, waited = 0;
 
 	flags = reduce_alloc_profile(extent_root, flags);
 
@@ -1530,7 +1530,18 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans,
 	    space_info->bytes_reserved + alloc_bytes) < thresh)
 		goto out;
 
-	mutex_lock(&extent_root->fs_info->chunk_mutex);
+	while (!mutex_trylock(&extent_root->fs_info->chunk_mutex)) {
+		if (!force)
+			goto out;
+		mutex_unlock(&extent_root->fs_info->alloc_mutex);
+		cond_resched();
+		mutex_lock(&extent_root->fs_info->alloc_mutex);
+		waited = 1;
+	}
+
+	if (waited && space_info->full)
+		goto out_unlock;
+
 	ret = btrfs_alloc_chunk(trans, extent_root, &start, &num_bytes, flags);
 	if (ret == -ENOSPC) {
 printk("space info full %Lu\n", flags);