On 3.05.2018 16:39, Jeff Mahoney wrote:
> On 5/3/18 3:24 AM, Nikolay Borisov wrote:
>>
>>
>> On 3.05.2018 00:11, jeffm@xxxxxxxx wrote:
>>> From: Jeff Mahoney <jeffm@xxxxxxxx>
>>>
>>> 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.
>>
>> Am I correct in my understanding that this qgroup_rescan_ready flag is
>> used to avoid qgroup_rescan_init being called AFTER it has already been
>> called but BEFORE queue_rescan_worker ? Why wasn't the initial version
>> of this patch without this flag sufficient?
>
> No, the race is between clearing the BTRFS_QGROUP_STATUS_FLAG_RESCAN
> flag near the end of the worker and clearing the running flag. The
> rescan lock is dropped in between, so btrfs_rescan_init will let a new
> rescan request in while we update the status item on disk. We wouldn't
> have queued another worker since that's what the warning catches, but if
> there were already tasks waiting for completion, they wouldn't have been
> woken since the wait queue list would be reinitialized. There's no way
> to reorder clearing the flag without changing how we handle
> ->qgroup_flags. I plan on doing that separately. This was just meant
> to be the simple fix.
Great, I think some of this information should go into the change log,
in explaining what the symptoms of the race condition are.
>
> That we can use the ready variable to also ensure that we don't let
> qgroup_rescan_init be called twice without running the rescan is a nice
> bonus.
>
> -Jeff
>
>>>
>>> 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@xxxxxxxx>
>>> ---
>>> 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)
>>> - btrfs_queue_work(fs_info->qgroup_rescan_workers,
>>> - &fs_info->qgroup_rescan_work);
>>> + queue_rescan_worker(fs_info);
>>> }
>>>
>>> /*
>>>
>
>
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