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

[Jeff Mahoney <jeffm@suse.com>]

On 4/30/18 2:20 AM, Qu Wenruo wrote:
> 
> 
> On 2018年04月27日 03:23, jeffm@suse.com wrote:
>> From: Jeff Mahoney <jeffm@suse.com>
>>
>> Commit d2c609b834d6 (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.
>>
>> This patch introduces a new helper, queue_rescan_worker, that handles
>> the ->qgroup_rescan_running flag, including any races with umount.
>>
>> 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: d2c609b834d6 (Btrfs: fix qgroup rescan worker initialization)
>> Signed-off-by: Jeff Mahoney <jeffm@suse.com>
> 
> A little off-topic, (thanks Nikolay for reporting this) sometimes
> btrfs/017 could report qgroup corruption, and it turns out it's related
> to rescan racy, which double account existing tree blocks twice.
> (One by btrfs quota enable, another by btrfs quota rescan -w)
> 
> Would this patch help in such case?

It shouldn't.  This only fixes races between the rescan worker getting
initialized and running vs waiting for it to complete.

-Jeff

>>  fs/btrfs/ctree.h  |  1 +
>>  fs/btrfs/qgroup.c | 40 ++++++++++++++++++++++++++++------------
>>  2 files changed, 29 insertions(+), 12 deletions(-)
>>
>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>> index da308774b8a4..dbba615f4d0f 100644
>> --- a/fs/btrfs/ctree.h
>> +++ b/fs/btrfs/ctree.h
>> @@ -1045,6 +1045,7 @@ 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_running;	/* protected by qgroup_rescan_lock */
>>  
>>  	/* filesystem state */
>> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
>> index aa259d6986e1..be491b6c020a 100644
>> --- a/fs/btrfs/qgroup.c
>> +++ b/fs/btrfs/qgroup.c
>> @@ -2072,6 +2072,30 @@ 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_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;
>> +	mutex_unlock(&fs_info->qgroup_rescan_lock);
>> +
>> +	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 +2147,7 @@ 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;
>>  	}
>> @@ -2713,7 +2736,6 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
>>  		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;
>>  
>>  	spin_unlock(&fs_info->qgroup_lock);
>>  	mutex_unlock(&fs_info->qgroup_rescan_lock);
>> @@ -2785,9 +2807,7 @@ 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 +2818,7 @@ 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 +2837,10 @@ 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