fs: reintroduce freezing nesting

PATCH In review Florian Margaine 2 participants 3 messages Apr 19, 2016 – Apr 22, 2016 VFS

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Florian Margaine

author

2016-04-19 20:47 UTC #0 lore raw

PATCH 1/1 fs: reintroduce freezing nesting

The behavior was removed in 18e9e5104fcd9a973ffe3eed3816c87f2a1b6cd2 noting that this was a better idea than using a counter. However, this behavior is actually wanted if multiple applications want to freeze concurrently while remaining non-racy.

This patch reintroduces this feature by using a counter.

---
fs/super.c | 15 +++++++++++----
include/linux/fs.h | 1 +

 2 files changed, 12 insertions(+), 4 deletions(-)

fs/super.c +0 -0

index 74914b1..9fa8ca1 100644
@@ -231,6 +231,7 @@ static struct super_block *alloc_super(struct

file_system_type *type, int flags)
*/
down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING); s->s_count = 1;
+ s->s_freezers = 0;
atomic_set(&s->s_active, 1); mutex_init(&s->s_vfs_rename_mutex); lockdep_set_class(&s->s_vfs_rename_mutex, &type-

Quoted 1 line

s_vfs_rename_key);

@@ -1275,12 +1276,12 @@ int freeze_super(struct super_block *sb) {
int ret;

+ sb->s_freezers++;
+ if (sb->s_writers.frozen != SB_UNFROZEN)
+ return 0;
+
atomic_inc(&sb->s_active); down_write(&sb->s_umount);
- if (sb->s_writers.frozen != SB_UNFROZEN) {
- deactivate_locked_super(sb);
- return -EBUSY;
- }

if (!(sb->s_flags & MS_BORN)) { up_write(&sb->s_umount);
@@ -1338,14 +1339,20 @@ EXPORT_SYMBOL(freeze_super);
* @sb: the super to thaw
*
* Unlocks the filesystem and marks it writeable again after freeze_super().
+ * Since nesting freezes is allowed, only the last freeze actually unlocks.
*/
int thaw_super(struct super_block *sb)
{
int error;

+ sb->s_freezers--;
+ if (sb->s_freezers > 0)
+ return 0;
+
down_write(&sb->s_umount);
if (sb->s_writers.frozen == SB_UNFROZEN) { up_write(&sb->s_umount);
+ sb->s_freezers++;
return -EINVAL;
}

include/linux/fs.h +0 -0

index e514f76..c045e2a 100644
@@ -1333,6 +1333,7 @@ struct super_block {

struct quota_info s_dquot; /* Diskquota specific
options */

struct sb_writers s_writers;
+ int s_freezers;

char s_id[32]; /* Informational
name */
u8 s_uuid[16]; /* UUID */
--
2.8.0

Mateusz Guzik

2016-04-19 21:39 UTC #1 lore raw

Quoted 7 lines · Florian Margaine

The behavior was removed in 18e9e5104fcd9a973ffe3eed3816c87f2a1b6cd2 noting that this was a better idea than using a counter. However, this behavior is actually wanted if multiple applications want to freeze concurrently while remaining non-racy.
This patch reintroduces this feature by using a counter.

This patch is wrong.

It uses non-atomic ops to modify the counter and no locks are held to protect it.

I would argue the code should track that freezing has started and additional freezers must only return when the state is SB_FREEZE_COMPLETE.

Quoted 80 lines

---
fs/super.c | 15 +++++++++++----
include/linux/fs.h | 1 +
 2 files changed, 12 insertions(+), 4 deletions(-)
fs/super.c +1 -0
index 74914b1..9fa8ca1 100644
@@ -231,6 +231,7 @@ static struct super_block *alloc_super(struct
 file_system_type *type, int flags)
  	 */
  	down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
  	s->s_count = 1;
+	s->s_freezers = 0;
  	atomic_set(&s->s_active, 1);
  	mutex_init(&s->s_vfs_rename_mutex);
lockdep_set_class(&s->s_vfs_rename_mutex, &type-
Quoted 1 line
s_vfs_rename_key);
fs/super.c +0 -0
@@ -1275,12 +1276,12 @@ int freeze_super(struct super_block *sb)
  {
  	int ret;
fs/super.c +4 -0
+	sb->s_freezers++;
+	if (sb->s_writers.frozen != SB_UNFROZEN)
+		return 0;
+
atomic_inc(&sb->s_active);
down_write(&sb->s_umount);
fs/super.c +0 -4
-	if (sb->s_writers.frozen != SB_UNFROZEN) {
-		deactivate_locked_super(sb);
-		return -EBUSY;
-	}
if (!(sb->s_flags & MS_BORN)) {
up_write(&sb->s_umount);
fs/super.c +0 -0
@@ -1338,14 +1339,20 @@ EXPORT_SYMBOL(freeze_super);
   * @sb: the super to thaw
   *
   * Unlocks the filesystem and marks it writeable again after
freeze_super().
+ * Since nesting freezes is allowed, only the last freeze actually unlocks.
*/
int thaw_super(struct super_block *sb)
{
int error;
fs/super.c +4 -0
+	sb->s_freezers--;
+	if (sb->s_freezers > 0)
+		return 0;
+
down_write(&sb->s_umount);
if (sb->s_writers.frozen == SB_UNFROZEN) { up_write(&sb->s_umount);
fs/super.c +1 -0
+		sb->s_freezers++;
return -EINVAL;
}
include/linux/fs.h +0 -0
index e514f76..c045e2a 100644
@@ -1333,6 +1333,7 @@ struct super_block {
struct quota_info s_dquot; /* Diskquota specific
options */

struct sb_writers s_writers;
include/linux/fs.h +1 -0
+	int			s_freezers;
char s_id[32]; /* Informational
name */
u8 s_uuid[16]; /* UUID */
--
2.8.0
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Signature · 1 line

Mateusz Guzik

Florian Margaine

author

2016-04-22 21:55 UTC #2 lore raw

Quoted 9 lines · Mateusz Guzik

Quoted 7 lines · Florian Margaine
The behavior was removed in 18e9e5104fcd9a973ffe3eed3816c87f2a1b6cd2 noting that this was a better idea than using a counter. However, this behavior is actually wanted if multiple applications want to freeze concurrently while remaining non-racy.
This patch reintroduces this feature by using a counter.

Quoted 8 lines

This patch is wrong.
It uses non-atomic ops to modify the counter and no locks are held to protect it.
I would argue the code should track that freezing has started and additional freezers must only return when the state is SB_FREEZE_COMPLETE.

Does it all additional freezers must return in order, or all additional freezers can return asap when SB_FREEZE_COMPLETE is set? I'm not sure the former is easily doable.

Quoted 84 lines

Quoted 77 lines

---
 fs/super.c         | 15 +++++++++++----
 include/linux/fs.h |  1 +
 2 files changed, 12 insertions(+), 4 deletions(-)

fs/super.c +1 -0

index 74914b1..9fa8ca1 100644
@@ -231,6 +231,7 @@ static struct super_block *alloc_super(struct
 file_system_type *type, int flags)
       */
      down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
      s->s_count = 1;
+     s->s_freezers = 0;
      atomic_set(&s->s_active, 1);
      mutex_init(&s->s_vfs_rename_mutex);

      lockdep_set_class(&s->s_vfs_rename_mutex, &type-

Quoted 1 line

s_vfs_rename_key);

fs/super.c +4 -4

@@ -1275,12 +1276,12 @@ int freeze_super(struct super_block *sb)
 {
      int ret;
 
+     sb->s_freezers++;
+     if (sb->s_writers.frozen != SB_UNFROZEN)
+             return 0;
+
      atomic_inc(&sb->s_active);
      down_write(&sb->s_umount);
-     if (sb->s_writers.frozen != SB_UNFROZEN) {
-             deactivate_locked_super(sb);
-             return -EBUSY;
-     }
 
      if (!(sb->s_flags & MS_BORN)) {
              up_write(&sb->s_umount);

@@ -1338,14 +1339,20 @@ EXPORT_SYMBOL(freeze_super);
  * @sb: the super to thaw
  *
  * Unlocks the filesystem and marks it writeable again after

freeze_super().
+ * Since nesting freezes is allowed, only the last freeze actually unlocks.
*/
int thaw_super(struct super_block *sb)

 {
      int error;

fs/super.c +5 -0

+     sb->s_freezers--;
+     if (sb->s_freezers > 0)
+             return 0;
+
      down_write(&sb->s_umount);
      if (sb->s_writers.frozen == SB_UNFROZEN) {
              up_write(&sb->s_umount);
+             sb->s_freezers++;
              return -EINVAL;
      }

include/linux/fs.h +0 -0

index e514f76..c045e2a 100644
@@ -1333,6 +1333,7 @@ struct super_block {
      struct quota_info       s_dquot;        /* Diskquota specific

options */

      struct sb_writers       s_writers;

+ int s_freezers;

char s_id[32]; /* Informational
name */
u8 s_uuid[16]; /* UUID */
--
2.8.0

Quoted 4 lines

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--
Mateusz Guzik

Signature · 1 line

Florian Margaine