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Date:   Thu, 31 Oct 2019 19:00:33 +0100
From:   Jan Kara <jack@...e.cz>
To:     Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        jack@...e.cz, Dmitry Monakhov <dmtrmonakhov@...dex-team.ru>
Subject: Re: [PATCH 2/2] fs/quota: Check that quota is not dirty before
 release

On Thu 31-10-19 10:39:20, Dmitry Monakhov wrote:
> From: Dmitry Monakhov <dmtrmonakhov@...dex-team.ru>
> 
> There is a race window where quota was redirted once we drop dq_list_lock inside dqput(),
> but before we grab dquot->dq_lock inside dquot_release()
> 
> TASK1                                                       TASK2 (chowner)
> ->dqput()
>   we_slept:
>     spin_lock(&dq_list_lock)
>     if (dquot_dirty(dquot)) {
>           spin_unlock(&dq_list_lock);
>           dquot->dq_sb->dq_op->write_dquot(dquot);
>           goto we_slept
>     if (test_bit(DQ_ACTIVE_B, &dquot->dq_flags)) {
>           spin_unlock(&dq_list_lock);
>           dquot->dq_sb->dq_op->release_dquot(dquot);
>                                                             dqget()
> 							    mark_dquot_dirty()
> 							    dqput()
>           goto we_slept;
>         }
> So dquot dirty quota will be released by TASK1, but on next we_sleept loop
> we detect this and call ->write_dquot() for it.
> XFSTEST: https://github.com/dmonakhov/xfstests/commit/440a80d4cbb39e9234df4d7240aee1d551c36107

Yeah, good catch. Both patches look good to me. I've added them to my tree.

								Honza

> 
> Signed-off-by: Dmitry Monakhov <dmtrmonakhov@...dex-team.ru>
> ---
>  fs/ocfs2/quota_global.c  |  2 +-
>  fs/quota/dquot.c         |  2 +-
>  include/linux/quotaops.h | 10 ++++++++++
>  3 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ocfs2/quota_global.c b/fs/ocfs2/quota_global.c
> index 7a92219..eda8348 100644
> --- a/fs/ocfs2/quota_global.c
> +++ b/fs/ocfs2/quota_global.c
> @@ -728,7 +728,7 @@ static int ocfs2_release_dquot(struct dquot *dquot)
>  
>  	mutex_lock(&dquot->dq_lock);
>  	/* Check whether we are not racing with some other dqget() */
> -	if (atomic_read(&dquot->dq_count) > 1)
> +	if (dquot_is_busy(dquot))
>  		goto out;
>  	/* Running from downconvert thread? Postpone quota processing to wq */
>  	if (current == osb->dc_task) {
> diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
> index b492b9e..72d24a5 100644
> --- a/fs/quota/dquot.c
> +++ b/fs/quota/dquot.c
> @@ -497,7 +497,7 @@ int dquot_release(struct dquot *dquot)
>  
>  	mutex_lock(&dquot->dq_lock);
>  	/* Check whether we are not racing with some other dqget() */
> -	if (atomic_read(&dquot->dq_count) > 1)
> +	if (dquot_is_busy(dquot))
>  		goto out_dqlock;
>  	if (dqopt->ops[dquot->dq_id.type]->release_dqblk) {
>  		ret = dqopt->ops[dquot->dq_id.type]->release_dqblk(dquot);
> diff --git a/include/linux/quotaops.h b/include/linux/quotaops.h
> index 185d948..91e0b76 100644
> --- a/include/linux/quotaops.h
> +++ b/include/linux/quotaops.h
> @@ -54,6 +54,16 @@ static inline struct dquot *dqgrab(struct dquot *dquot)
>  	atomic_inc(&dquot->dq_count);
>  	return dquot;
>  }
> +
> +static inline bool dquot_is_busy(struct dquot *dquot)
> +{
> +	if (test_bit(DQ_MOD_B, &dquot->dq_flags))
> +		return true;
> +	if (atomic_read(&dquot->dq_count) > 1)
> +		return true;
> +	return false;
> +}
> +
>  void dqput(struct dquot *dquot);
>  int dquot_scan_active(struct super_block *sb,
>  		      int (*fn)(struct dquot *dquot, unsigned long priv),
> -- 
> 2.7.4
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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