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SubjectRe: [RFC PATCH 1/4] vfs: Create fs_context-aware mount_bdev() replacement
Date
Andrew Price <anprice@redhat.com> wrote:

> > + up_write(&s->s_umount);
> > + blkdev_put(bdev, fc->bdev_mode);
> > + down_write(&s->s_umount);
>
> fc->bdev should be NULLed here (or, on the way out of sget_fc() might be more
> appropriate) otherwise we get a double-blkdev_put() leading to NULL pointer
> derefs later. This happens when I mount a device twice and then unmount them,
> or mount it 3 times.

How about the attached changes? Note that they conflict a bit with the mtd
changes from where I took the destructor piece.

David
---
commit 161602f963ae5a05bdb834461f7a4896286fbde0
Author: David Howells <dhowells@redhat.com>
Date: Wed Mar 27 11:12:57 2019 +0000

bdev handling fix

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 9e22fe6aafe7..fc8fda4b9fe9 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -425,10 +425,8 @@ void put_fs_context(struct fs_context *fc)

if (fc->need_free && fc->ops && fc->ops->free)
fc->ops->free(fc);
-#ifdef CONFIG_BLOCK
- if (fc->bdev)
- blkdev_put(fc->bdev, fc->bdev_mode);
-#endif
+ if (fc->dev_destructor)
+ fc->dev_destructor(fc);

security_free_mnt_opts(&fc->security);
put_net(fc->net_ns);
diff --git a/fs/super.c b/fs/super.c
index 85851adb0f19..e9e678d2c37c 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1211,8 +1211,17 @@ int vfs_get_super(struct fs_context *fc,
EXPORT_SYMBOL(vfs_get_super);

#ifdef CONFIG_BLOCK
+static void fc_bdev_destructor(struct fs_context *fc)
+{
+ if (fc->bdev) {
+ blkdev_put(fc->bdev, fc->bdev_mode);
+ fc->bdev = NULL;
+ }
+}
+
static int set_bdev_super_fc(struct super_block *s, struct fs_context *fc)
{
+ s->s_mode = fc->bdev_mode;
s->s_bdev = fc->bdev;
s->s_dev = s->s_bdev->bd_dev;
s->s_bdi = bdi_get(s->s_bdev->bd_bdi);
@@ -1252,6 +1261,9 @@ int vfs_get_block_super(struct fs_context *fc,
return PTR_ERR(bdev);
}

+ fc->dev_destructor = fc_bdev_destructor;
+ fc->bdev = bdev;
+
/* Once the superblock is inserted into the list by sget_fc(), s_umount
* will protect the lockfs code from trying to start a snapshot while
* we are mounting
@@ -1260,18 +1272,14 @@ int vfs_get_block_super(struct fs_context *fc,
if (bdev->bd_fsfreeze_count > 0) {
mutex_unlock(&bdev->bd_fsfreeze_mutex);
warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
- error = -EBUSY;
- goto error_bdev;
+ return -EBUSY;
}

- fc->bdev = bdev;
fc->sb_flags |= SB_NOSEC;
s = sget_fc(fc, test_bdev_super_fc, set_bdev_super_fc);
mutex_unlock(&bdev->bd_fsfreeze_mutex);
- if (IS_ERR(s)) {
- error = PTR_ERR(s);
- goto error_bdev;
- }
+ if (IS_ERR(s))
+ return PTR_ERR(s);

if (s->s_root) {
/* Don't summarily change the RO/RW state. */
@@ -1281,16 +1289,10 @@ int vfs_get_block_super(struct fs_context *fc,
goto error_sb;
}

- /* s_umount nests inside bd_mutex during __invalidate_device().
- * blkdev_put() acquires bd_mutex and can't be called under
- * s_umount. Drop s_umount temporarily. This is safe as we're
- * holding an active reference.
+ /* Leave fc->bdev to fc_bdev_destructor() to clean up to avoid
+ * locking conflicts.
*/
- up_write(&s->s_umount);
- blkdev_put(bdev, fc->bdev_mode);
- down_write(&s->s_umount);
} else {
- s->s_mode = fc->bdev_mode;
snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
sb_set_blocksize(s, block_size(bdev));
error = fill_super(s, fc);
@@ -1307,11 +1309,7 @@ int vfs_get_block_super(struct fs_context *fc,

error_sb:
deactivate_locked_super(s);
-error_bdev:
- if (fc->bdev) {
- blkdev_put(fc->bdev, fc->bdev_mode);
- fc->bdev = NULL;
- }
+ /* Leave fc->bdev to fc_bdev_destructor() to clean up */
return error;
}
EXPORT_SYMBOL(vfs_get_block_super);
@@ -1521,8 +1519,13 @@ int vfs_get_tree(struct fs_context *fc)
* on the superblock.
*/
error = fc->ops->get_tree(fc);
- if (error < 0)
+ if (error < 0) {
+ if (fc->dev_destructor) {
+ fc->dev_destructor(fc);
+ fc->dev_destructor = NULL;
+ }
return error;
+ }

if (!fc->root) {
pr_err("Filesystem %s get_tree() didn't set fc->root\n",
diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
index cb49b92f02af..9af788a3ef8e 100644
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -76,7 +76,9 @@ struct fs_context {
const struct fs_context_operations *ops;
struct file_system_type *fs_type;
void *fs_private; /* The filesystem's context */
- struct block_device *bdev; /* The backing blockdev (if applicable) */
+ union {
+ struct block_device *bdev; /* The backing blockdev (if applicable) */
+ };
struct dentry *root; /* The root and superblock */
struct user_namespace *user_ns; /* The user namespace for this mount */
struct net *net_ns; /* The network namespace for this mount */
@@ -93,6 +95,7 @@ struct fs_context {
enum fs_context_purpose purpose:8;
bool need_free:1; /* Need to call ops->free() */
bool global:1; /* Goes into &init_user_ns */
+ void (*dev_destructor)(struct fs_context *fc); /* For block or mtd */
};

struct fs_context_operations {
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