Wang Shilong
2014-Jun-11 12:11 UTC
[PATCH] Btrfs-progs: fix race condition between btrfs and udev
Originally this problem was reproduced by the following scripts:
# dd if=/dev/zero of=data bs=1M count=50
# losetup /dev/loop1 data
# i =1
# while [ 1 ]
do
mkfs.btrfs -fK /dev/loop1 >& /dev/null || exit 1
((i++))
echo "loop $i"
done
Futher, a easy way to trigger this problem is by running the following
c codes repeatedly:
int main(int argc, char **argv)
{
/* pass a btrfs block device */
int fd = open(argv[1], O_RDWR | O_EXCL);
if (fd < 0) {
perror("fail to open: %s", strerror(errno));
exit(1);
}
close(fd);
return 0;
}
So the problem is RW opening would trigger udev event which will
call btrfs_scan_one_device(). In btrfs_scan_one_device(), it
would open the block device with EXCL flag..meanwhile if another
program try to open that device with O_EXCL, it would fail with
EBUSY.
This happen seldomly in the real world, but if we use loop device
for test, we may hit this annoying problem.
A walkaround way to solve this problem is to wait kernel scanning
finished and then try it again.
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
---
utils.c | 36 +++++++++++++++++++++++++++++++++++-
utils.h | 1 +
volumes.c | 3 ++-
3 files changed, 38 insertions(+), 2 deletions(-)
diff --git a/utils.c b/utils.c
index d29de94..20108c3 100644
--- a/utils.c
+++ b/utils.c
@@ -2058,6 +2058,40 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64
data_profile,
return 0;
}
+/*
+ * there is a race condition between btrfs and udev, we may fail
+ * because udev call btrfs_scan_device() which will open the block
+ * device with O_EXCL. A walkaround solution is to wait kernel
+ * scanning finished and then try again.
+ */
+int btrfs_open_block_device(char *file, int flag)
+{
+ int ret, fd;
+ struct btrfs_ioctl_vol_args args;
+ int tried = 0;
+
+again:
+ fd = open(file, flag);
+ if (fd < 0) {
+ if (!tried && errno == EBUSY && (flag & O_EXCL)) {
+ tried = 1;
+ fd = open("/dev/btrfs-control", O_RDWR);
+ if (fd < 0) {
+ fprintf(stderr,
+ "unable to open /dev/btrfs-control: %s\n",
+ strerror(errno));
+ return fd;
+ }
+ strncpy(args.name, file, BTRFS_PATH_NAME_MAX);
+ ret = ioctl(fd, BTRFS_IOC_SCAN_DEV, &args);
+ close(fd);
+ if (!ret)
+ goto again;
+ }
+ }
+ return fd;
+}
+
/* Check if disk is suitable for btrfs
* returns:
* 1: something is wrong, estr provides the error
@@ -2096,7 +2130,7 @@ int test_dev_for_mkfs(char *file, int force_overwrite,
char *estr)
return 1;
}
/* check if the device is busy */
- fd = open(file, O_RDWR|O_EXCL);
+ fd = btrfs_open_block_device(file, O_RDWR | O_EXCL);
if (fd < 0) {
snprintf(estr, sz, "unable to open %s: %s\n", file,
strerror(errno));
diff --git a/utils.h b/utils.h
index 0b03830..f6ca252 100644
--- a/utils.h
+++ b/utils.h
@@ -95,6 +95,7 @@ int open_path_or_dev_mnt(const char *path, DIR **dirstream);
u64 btrfs_device_size(int fd, struct stat *st);
/* Helper to always get proper size of the destination string */
#define strncpy_null(dest, src) __strncpy__null(dest, src, sizeof(dest))
+int btrfs_open_block_device(char *file, int flag);
int test_dev_for_mkfs(char *file, int force_overwrite, char *estr);
int scan_for_btrfs(int where, int update_kernel);
int get_label_mounted(const char *mount_path, char *labelp);
diff --git a/volumes.c b/volumes.c
index a61928c..f40740a 100644
--- a/volumes.c
+++ b/volumes.c
@@ -30,6 +30,7 @@
#include "print-tree.h"
#include "volumes.h"
#include "math.h"
+#include "utils.h"
struct stripe {
struct btrfs_device *dev;
@@ -207,7 +208,7 @@ int btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
int flags)
continue;
}
- fd = open(device->name, flags);
+ fd = btrfs_open_block_device(device->name, flags);
if (fd < 0) {
ret = -errno;
goto fail;
--
1.9.0
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