[PATCH] btrfs-progs: Fix data races in btrfs-image.

[Adam Buchbinder <abuchbinder@google.com>]

Making the code data-race safe requires that reads *and* writes
happen under a mutex lock, if any of the access are writes. See
Dmitri Vyukov, "Benign data races: what could possibly go wrong?"
for more details.

The fix here was to put most of the main loop of restore_worker
under a mutex lock.

This race was detected using fsck-tests/012-leaf-corruption.

==================
WARNING: ThreadSanitizer: data race
  Write of size 4 by main thread:
    #0 add_cluster btrfs-progs/image/main.c:1931
    #1 restore_metadump btrfs-progs/image/main.c:2566
    #2 main btrfs-progs/image/main.c:2859

  Previous read of size 4 by thread T6:
    #0 restore_worker btrfs-progs/image/main.c:1720

  Location is stack of main thread.

  Thread T6 (running) created by main thread at:
    #0 pthread_create <null>
    #1 mdrestore_init btrfs-progs/image/main.c:1868
    #2 restore_metadump btrfs-progs/image/main.c:2534
    #3 main btrfs-progs/image/main.c:2859

SUMMARY: ThreadSanitizer: data race btrfs-progs/image/main.c:1931 in
add_cluster

Signed-off-by: Adam Buchbinder <abuchbinder@google.com>
---
 image/main.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/image/main.c b/image/main.c
index 1eca414..a5d01d8 100644
--- a/image/main.c
+++ b/image/main.c
@@ -1715,14 +1715,15 @@ static void *restore_worker(void *data)
 		}
 		async = list_entry(mdres->list.next, struct async_work, list);
 		list_del_init(&async->list);
-		pthread_mutex_unlock(&mdres->mutex);
 
 		if (mdres->compress_method == COMPRESS_ZLIB) {
 			size = compress_size; 
+			pthread_mutex_unlock(&mdres->mutex);
 			ret = uncompress(buffer, (unsigned long *)&size,
 					 async->buffer, async->bufsize);
+			pthread_mutex_lock(&mdres->mutex);
 			if (ret != Z_OK) {
-				error("decompressiion failed with %d", ret);
+				error("decompression failed with %d", ret);
 				err = -EIO;
 			}
 			outbuf = buffer;
@@ -1798,7 +1799,6 @@ error:
 		if (!mdres->multi_devices && async->start == BTRFS_SUPER_INFO_OFFSET)
 			write_backup_supers(outfd, outbuf);
 
-		pthread_mutex_lock(&mdres->mutex);
 		if (err && !mdres->error)
 			mdres->error = err;
 		mdres->num_items--;
@@ -1899,7 +1899,7 @@ static int fill_mdres_info(struct mdrestore_struct *mdres,
 		ret = uncompress(buffer, (unsigned long *)&size,
 				 async->buffer, async->bufsize);
 		if (ret != Z_OK) {
-			error("decompressiion failed with %d", ret);
+			error("decompression failed with %d", ret);
 			free(buffer);
 			return -EIO;
 		}
@@ -1928,7 +1928,9 @@ static int add_cluster(struct meta_cluster *cluster,
 	u32 i, nritems;
 	int ret;
 
+	pthread_mutex_lock(&mdres->mutex);
 	mdres->compress_method = header->compress;
+	pthread_mutex_unlock(&mdres->mutex);
 
 	bytenr = le64_to_cpu(header->bytenr) + BLOCK_SIZE;
 	nritems = le32_to_cpu(header->nritems);
@@ -2171,7 +2173,7 @@ static int search_for_chunk_blocks(struct mdrestore_struct *mdres,
 				continue;
 			}
 			error(
-	"unknown state after reading cluster at %llu, probably crrupted data",
+	"unknown state after reading cluster at %llu, probably corrupted data",
 					cluster_bytenr);
 			ret = -EIO;
 			break;
@@ -2220,7 +2222,7 @@ static int search_for_chunk_blocks(struct mdrestore_struct *mdres,
 						 (unsigned long *)&size, tmp,
 						 bufsize);
 				if (ret != Z_OK) {
-					error("decompressiion failed with %d",
+					error("decompression failed with %d",
 							ret);
 					ret = -EIO;
 					break;
@@ -2340,7 +2342,7 @@ static int build_chunk_tree(struct mdrestore_struct *mdres,
 		ret = uncompress(tmp, (unsigned long *)&size,
 				 buffer, le32_to_cpu(item->size));
 		if (ret != Z_OK) {
-			error("decompressiion failed with %d", ret);
+			error("decompression failed with %d", ret);
 			free(buffer);
 			free(tmp);
 			return -EIO;
-- 
2.13.2.932.g7449e964c-goog