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From: Jeff Mahoney <jeffm@suse.com>
To: dsterba@suse.com, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 1/3] btrfs: qgroups, fix rescan worker running races
Date: Thu, 10 May 2018 19:04:12 -0400	[thread overview]
Message-ID: <a3bb4d77-3783-219a-fad2-3c31ec0c8b38@suse.com> (raw)
In-Reply-To: <20180502211156.9460-2-jeffm@suse.com>


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On 5/2/18 5:11 PM, jeffm@suse.com wrote:
> From: Jeff Mahoney <jeffm@suse.com>
> 
> Commit 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization)
> fixed the issue with BTRFS_IOC_QUOTA_RESCAN_WAIT being racy, but
> ended up reintroducing the hang-on-unmount bug that the commit it
> intended to fix addressed.
> 
> The race this time is between qgroup_rescan_init setting
> ->qgroup_rescan_running = true and the worker starting.  There are
> many scenarios where we initialize the worker and never start it.  The
> completion btrfs_ioctl_quota_rescan_wait waits for will never come.
> This can happen even without involving error handling, since mounting
> the file system read-only returns between initializing the worker and
> queueing it.
> 
> The right place to do it is when we're queuing the worker.  The flag
> really just means that btrfs_ioctl_quota_rescan_wait should wait for
> a completion.
> 
> Since the BTRFS_QGROUP_STATUS_FLAG_RESCAN flag is overloaded to
> refer to both runtime behavior and on-disk state, we introduce a new
> fs_info->qgroup_rescan_ready to indicate that we're initialized and
> waiting to start.
> 
> This patch introduces a new helper, queue_rescan_worker, that handles
> most of the initialization, the two flags, and queuing the worker,
> including races with unmount.
> 
> While we're at it, ->qgroup_rescan_running is protected only by the
> ->qgroup_rescan_mutex.  btrfs_ioctl_quota_rescan_wait doesn't need
> to take the spinlock too.
> 
> Fixes: 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization)
> Signed-off-by: Jeff Mahoney <jeffm@suse.com>
> ---
>  fs/btrfs/ctree.h  |  2 ++
>  fs/btrfs/qgroup.c | 94 +++++++++++++++++++++++++++++++++----------------------
>  2 files changed, 58 insertions(+), 38 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index da308774b8a4..4003498bb714 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1045,6 +1045,8 @@ struct btrfs_fs_info {
>  	struct btrfs_workqueue *qgroup_rescan_workers;
>  	struct completion qgroup_rescan_completion;
>  	struct btrfs_work qgroup_rescan_work;
> +	/* qgroup rescan worker is running or queued to run */
> +	bool qgroup_rescan_ready;
>  	bool qgroup_rescan_running;	/* protected by qgroup_rescan_lock */
>  
>  	/* filesystem state */
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index aa259d6986e1..466744741873 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -101,6 +101,7 @@ static int
>  qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
>  		   int init_flags);
>  static void qgroup_rescan_zero_tracking(struct btrfs_fs_info *fs_info);
> +static void btrfs_qgroup_rescan_worker(struct btrfs_work *work);
>  
>  /* must be called with qgroup_ioctl_lock held */
>  static struct btrfs_qgroup *find_qgroup_rb(struct btrfs_fs_info *fs_info,
> @@ -2072,6 +2073,46 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans,
>  	return ret;
>  }
>  
> +static void queue_rescan_worker(struct btrfs_fs_info *fs_info)
> +{
> +	mutex_lock(&fs_info->qgroup_rescan_lock);
> +	if (btrfs_fs_closing(fs_info)) {
> +		mutex_unlock(&fs_info->qgroup_rescan_lock);
> +		return;
> +	}
> +
> +	if (WARN_ON(!fs_info->qgroup_rescan_ready)) {
> +		btrfs_warn(fs_info, "rescan worker not ready");
> +		mutex_unlock(&fs_info->qgroup_rescan_lock);
> +		return;
> +	}
> +	fs_info->qgroup_rescan_ready = false;
> +
> +	if (WARN_ON(fs_info->qgroup_rescan_running)) {
> +		btrfs_warn(fs_info, "rescan worker already queued");
> +		mutex_unlock(&fs_info->qgroup_rescan_lock);
> +		return;
> +	}
> +
> +	/*
> +	 * Being queued is enough for btrfs_qgroup_wait_for_completion
> +	 * to need to wait.
> +	 */
> +	fs_info->qgroup_rescan_running = true;
> +	init_completion(&fs_info->qgroup_rescan_completion);
> +	mutex_unlock(&fs_info->qgroup_rescan_lock);
> +
> +	memset(&fs_info->qgroup_rescan_work, 0,
> +	       sizeof(fs_info->qgroup_rescan_work));
> +
> +	btrfs_init_work(&fs_info->qgroup_rescan_work,
> +			btrfs_qgroup_rescan_helper,
> +			btrfs_qgroup_rescan_worker, NULL, NULL);
> +
> +	btrfs_queue_work(fs_info->qgroup_rescan_workers,
> +			 &fs_info->qgroup_rescan_work);
> +}
> +
>  /*
>   * called from commit_transaction. Writes all changed qgroups to disk.
>   */
> @@ -2123,8 +2164,7 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans,
>  		ret = qgroup_rescan_init(fs_info, 0, 1);
>  		if (!ret) {
>  			qgroup_rescan_zero_tracking(fs_info);
> -			btrfs_queue_work(fs_info->qgroup_rescan_workers,
> -					 &fs_info->qgroup_rescan_work);
> +			queue_rescan_worker(fs_info);
>  		}
>  		ret = 0;
>  	}
> @@ -2607,6 +2647,10 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
>  	if (!path)
>  		goto out;
>  
> +	mutex_lock(&fs_info->qgroup_rescan_lock);
> +	fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
> +	mutex_unlock(&fs_info->qgroup_rescan_lock);
> +
>  	err = 0;
>  	while (!err && !btrfs_fs_closing(fs_info)) {
>  		trans = btrfs_start_transaction(fs_info->fs_root, 0);
> @@ -2685,47 +2729,27 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
>  {
>  	int ret = 0;
>  
> -	if (!init_flags &&
> -	    (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) ||
> -	     !(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))) {
> +	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags)) {
>  		ret = -EINVAL;
>  		goto err;
>  	}
>  
>  	mutex_lock(&fs_info->qgroup_rescan_lock);
> -	spin_lock(&fs_info->qgroup_lock);
> -
> -	if (init_flags) {
> -		if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
> -			ret = -EINPROGRESS;
> -		else if (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))
> -			ret = -EINVAL;
> -
> -		if (ret) {
> -			spin_unlock(&fs_info->qgroup_lock);
> -			mutex_unlock(&fs_info->qgroup_rescan_lock);
> -			goto err;
> -		}
> -		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
> +	if (fs_info->qgroup_rescan_ready || fs_info->qgroup_rescan_running) {
> +		mutex_unlock(&fs_info->qgroup_rescan_lock);
> +		ret = -EINPROGRESS;
> +		goto err;
>  	}

Without checking for these flags when deciding whether we want to do
accounting, we'll end up doing the accounting until we start the rescan
thread.  This may not matter normally, but I'm working on a patch as a
workaround to 824d8dff8846533c9f1f9b1eabb0c03959e989ca doing all qgroup
accounting for entire trees in single transactions.

-Jeff

>  	memset(&fs_info->qgroup_rescan_progress, 0,
>  		sizeof(fs_info->qgroup_rescan_progress));
>  	fs_info->qgroup_rescan_progress.objectid = progress_objectid;
> -	init_completion(&fs_info->qgroup_rescan_completion);
> -	fs_info->qgroup_rescan_running = true;
> +	fs_info->qgroup_rescan_ready = true;
>  
> -	spin_unlock(&fs_info->qgroup_lock);
>  	mutex_unlock(&fs_info->qgroup_rescan_lock);
>  
> -	memset(&fs_info->qgroup_rescan_work, 0,
> -	       sizeof(fs_info->qgroup_rescan_work));
> -	btrfs_init_work(&fs_info->qgroup_rescan_work,
> -			btrfs_qgroup_rescan_helper,
> -			btrfs_qgroup_rescan_worker, NULL, NULL);
> -
> -	if (ret) {
>  err:
> +	if (ret) {
>  		btrfs_info(fs_info, "qgroup_rescan_init failed with %d", ret);
>  		return ret;
>  	}
> @@ -2785,9 +2809,7 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
>  
>  	qgroup_rescan_zero_tracking(fs_info);
>  
> -	btrfs_queue_work(fs_info->qgroup_rescan_workers,
> -			 &fs_info->qgroup_rescan_work);
> -
> +	queue_rescan_worker(fs_info);
>  	return 0;
>  }
>  
> @@ -2798,9 +2820,7 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
>  	int ret = 0;
>  
>  	mutex_lock(&fs_info->qgroup_rescan_lock);
> -	spin_lock(&fs_info->qgroup_lock);
>  	running = fs_info->qgroup_rescan_running;
> -	spin_unlock(&fs_info->qgroup_lock);
>  	mutex_unlock(&fs_info->qgroup_rescan_lock);
>  
>  	if (!running)
> @@ -2819,12 +2839,10 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
>   * this is only called from open_ctree where we're still single threaded, thus
>   * locking is omitted here.
>   */
> -void
> -btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
> +void btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
>  {
>  	if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
> -		btrfs_queue_work(fs_info->qgroup_rescan_workers,
> -				 &fs_info->qgroup_rescan_work);
> +		queue_rescan_worker(fs_info);
>  }
>  
>  /*
> 


-- 
Jeff Mahoney
SUSE Labs


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  parent reply	other threads:[~2018-05-10 23:04 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-02 21:11 [PATCH v3 0/3] btrfs: qgroup rescan races (part 1) jeffm
2018-05-02 21:11 ` [PATCH 1/3] btrfs: qgroups, fix rescan worker running races jeffm
2018-05-03  7:24   ` Nikolay Borisov
2018-05-03 13:39     ` Jeff Mahoney
2018-05-03 15:52       ` Nikolay Borisov
2018-05-03 15:57         ` Jeff Mahoney
2018-05-10 19:49   ` Jeff Mahoney
2018-05-10 23:04   ` Jeff Mahoney [this message]
2020-01-16  6:41   ` Qu Wenruo
2018-05-02 21:11 ` [PATCH 2/3] btrfs: qgroups, remove unnecessary memset before btrfs_init_work jeffm
2018-05-02 21:11 ` [PATCH 3/3] btrfs: qgroup, don't try to insert status item after ENOMEM in rescan worker jeffm
2018-05-03  6:23 ` [PATCH v3 0/3] btrfs: qgroup rescan races (part 1) Nikolay Borisov
2018-05-03 22:27   ` Jeff Mahoney
2018-05-04  5:59     ` Nikolay Borisov
2018-05-04 13:32       ` Jeff Mahoney
2018-05-04 13:41         ` Nikolay Borisov
2019-11-28  3:28 ` Qu Wenruo
2019-12-03 19:32   ` David Sterba
  -- strict thread matches above, loose matches on Subject: below --
2018-04-26 19:23 [PATCH 1/3] btrfs: qgroups, fix rescan worker running races jeffm
2018-04-27  8:42 ` Nikolay Borisov
2018-04-27  8:48 ` Filipe Manana
2018-04-27 16:00   ` Jeff Mahoney
2018-04-27 15:56 ` David Sterba
2018-04-27 16:02   ` Jeff Mahoney
2018-04-27 16:40     ` David Sterba
2018-04-27 19:32       ` Jeff Mahoney
2018-04-28 17:09         ` David Sterba
2018-04-27 19:28   ` Noah Massey
2018-04-28 17:10     ` David Sterba
2018-04-30  6:20 ` Qu Wenruo
2018-04-30 14:07   ` Jeff Mahoney
2018-05-02 10:29 ` David Sterba
2018-05-02 13:15   ` David Sterba
2018-05-02 13:58     ` Jeff Mahoney

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