[PATCH 2/2] btrfs: scrub: move scrub_setup_ctx allocation out of device_list_mutex

[David Sterba <dsterba@xxxxxxxx>]

The scrub context is allocated with GFP_KERNEL and called from
btrfs_scrub_dev under the fs_info::device_list_mutex. This is not safe
regarding reclaim that could try to flush filesystem data in order to
get the memory. And the device_list_mutex is held during superblock
commit, so this would cause a lockup.

Move the alocation and initialization before any changes that require
the mutex.

Signed-off-by: David Sterba <dsterba@xxxxxxxx>
---
 fs/btrfs/scrub.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index ffcab263e057..051d14c9f013 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -3834,13 +3834,18 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 		return -EINVAL;
 	}
 
+	/* Allocate outside of device_list_mutex */
+	sctx = scrub_setup_ctx(fs_info, is_dev_replace);
+	if (IS_ERR(sctx))
+		return PTR_ERR(sctx);
 
 	mutex_lock(&fs_info->fs_devices->device_list_mutex);
 	dev = btrfs_find_device(fs_info, devid, NULL, NULL);
 	if (!dev || (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state) &&
 		     !is_dev_replace)) {
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
-		return -ENODEV;
+		ret = -ENODEV;
+		goto out_free_ctx;
 	}
 
 	if (!is_dev_replace && !readonly &&
@@ -3848,7 +3853,8 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
 		btrfs_err_in_rcu(fs_info, "scrub: device %s is not writable",
 				rcu_str_deref(dev->name));
-		return -EROFS;
+		ret = -EROFS;
+		goto out_free_ctx;
 	}
 
 	mutex_lock(&fs_info->scrub_lock);
@@ -3856,7 +3862,8 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 	    test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &dev->dev_state)) {
 		mutex_unlock(&fs_info->scrub_lock);
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
-		return -EIO;
+		ret = -EIO;
+		goto out_free_ctx;
 	}
 
 	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
@@ -3866,7 +3873,8 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 		btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
 		mutex_unlock(&fs_info->scrub_lock);
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
-		return -EINPROGRESS;
+		ret = -EINPROGRESS;
+		goto out_free_ctx;
 	}
 	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
 
@@ -3874,16 +3882,9 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 	if (ret) {
 		mutex_unlock(&fs_info->scrub_lock);
 		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
-		return ret;
+		goto out_free_ctx;
 	}
 
-	sctx = scrub_setup_ctx(fs_info, is_dev_replace);
-	if (IS_ERR(sctx)) {
-		mutex_unlock(&fs_info->scrub_lock);
-		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
-		scrub_workers_put(fs_info);
-		return PTR_ERR(sctx);
-	}
 	sctx->readonly = readonly;
 	dev->scrub_ctx = sctx;
 	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
@@ -3936,6 +3937,11 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 
 	scrub_put_ctx(sctx);
 
+	return ret;
+
+out_free_ctx:
+	scrub_free_ctx(sctx);
+
 	return ret;
 }
 
-- 
2.19.1