btrfs-progs: image: fix infinite looping with -t 0

Bug:
 # btrfs-image -t0 -c9 /dev/sda6 /tmp/btrfs_image.img
 (hang)
 # btrfs-image -r -t0 /tmp/btrfs_image.img /dev/sda6
 (hang)

Reason:
 The program need to create at least 1 thread for
 compression/decompression. If the user specifies -t0, it overwrites the
 default value of 1, then the program really creates 0 thread, and
 ends up looping

Fix:
 We can add a check, to make the program not to allow -t0 argument,
 but there is another problem:

For example, on a node with 4 cpus:
 btrfs-image -c9 -t1: 4 threads (1 means use NR_CPUS)
             -c9 -t2: 2 threads
             -c9 -t3: 3 threads
             ...
 (-t1 has more threads than -t2 and -t3)

 So we change to use value of 0 as "use NR_CPUS threads", then:
 btrfs-image [no -t arg]: use NR_CPUS threads
             -t0:         use NR_CPUS threads
             -t val:      use val threads.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/btrfs-image.c b/btrfs-image.c
index 630fdf8..6eddbd5 100644
--- a/btrfs-image.c
+++ b/btrfs-image.c
@@ -2690,7 +2690,7 @@ int main(int argc, char *argv[])
 {
 	char *source;
 	char *target;
-	u64 num_threads = 1;
+	u64 num_threads = 0;
 	u64 compress_level = 0;
 	int create = 1;
 	int old_restore = 0;
@@ -2786,7 +2786,8 @@ int main(int argc, char *argv[])
 		}
 	}
 
-	if (num_threads == 1 && compress_level > 0) {
+	if ((compress_level > 0 || create == 0) &&
+	    num_threads == 0) {
 		num_threads = sysconf(_SC_NPROCESSORS_ONLN);
 		if (num_threads <= 0)
 			num_threads = 1;