On 26.04.2018 22:23, jeffm@xxxxxxxx wrote:
> From: Jeff Mahoney <jeffm@xxxxxxxx>
>
> 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@xxxxxxxx>
LGTM.
Reviewed-by: Nikolay Borisov <nborisov@xxxxxxxx>
> ---
> 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);
> }
So here it's not possible to race, since if qgroup_rescan_init returns 0
then we are guaranteed to queue the rescan.
> 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);
Which leaves this to be the only problematic case, in case transaction
joining/commit fails, right?
> 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);
> }
>
> /*
>
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