On Tue, Apr 19, 2016 at 10:48:41PM +0200, Florian Margaine wrote:
> 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.
> ---
> fs/super.c | 15 +++++++++++----
> include/linux/fs.h | 1 +
> 2 files changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/fs/super.c b/fs/super.c
> index 74914b1..9fa8ca1 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -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-
> >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;
> }
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index e514f76..c045e2a 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -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
>
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--
Mateusz Guzik
