Re: [PATCH 1/3] btrfs: qgroups, fix rescan worker running races

[Jeff Mahoney <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;
>  	}
>  
>  	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)

This check will never be true since the worker is now responsible for
setting it.

-Jeff

> -		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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