linux-kernel - Re: [PATCH] fs: reintroduce freezing nesting

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Date:	Fri, 22 Apr 2016 23:54:51 +0200
From:	Florian Margaine <florian@...gaine.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-fsdevel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs: reintroduce freezing nesting

On Tue, Apr 19, 2016 at 11:39 PM, Mateusz Guzik <mguzik@...hat.com> wrote:
> 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.

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.

>
>> ---
>>  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



-- 
Florian Margaine