ublk: scan partition in async way

Implement async partition scan to avoid IO hang when reading partition
tables. Similar to nvme_partition_scan_work(), partition scanning is
deferred to a work queue to prevent deadlocks.

When partition scan happens synchronously during add_disk(), IO errors
can cause the partition scan to wait while holding ub->mutex, which
can deadlock with other operations that need the mutex.

Changes:
- Add partition_scan_work to ublk_device structure
- Implement ublk_partition_scan_work() to perform async scan
- Always suppress sync partition scan during add_disk()
- Schedule async work after add_disk() for trusted daemons
- Add flush_work() in ublk_stop_dev() before grabbing ub->mutex

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Reported-by: Yoav Cohen <yoav@nvidia.com>
Closes: https://lore.kernel.org/linux-block/DM4PR12MB63280C5637917C071C2F0D65A9A8A@DM4PR12MB6328.namprd12.prod.outlook.com/
Fixes: 71f28f3136af ("ublk_drv: add io_uring based userspace block driver")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 49c20845719815bdf0429c5740a3c80b5bf19497..837fedb02e0d58ee45933c3f8e271694e27204e7 100644 (file)
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -237,6 +237,7 @@ struct ublk_device {
        bool canceling;
        pid_t   ublksrv_tgid;
        struct delayed_work     exit_work;
+       struct work_struct      partition_scan_work;
 
        struct ublk_queue       *queues[];
 };
@@ -254,6 +255,20 @@ static inline struct request *__ublk_check_and_get_req(struct ublk_device *ub,
                u16 q_id, u16 tag, struct ublk_io *io, size_t offset);
 static inline unsigned int ublk_req_build_flags(struct request *req);
 
+static void ublk_partition_scan_work(struct work_struct *work)
+{
+       struct ublk_device *ub =
+               container_of(work, struct ublk_device, partition_scan_work);
+
+       if (WARN_ON_ONCE(!test_and_clear_bit(GD_SUPPRESS_PART_SCAN,
+                                            &ub->ub_disk->state)))
+               return;
+
+       mutex_lock(&ub->ub_disk->open_mutex);
+       bdev_disk_changed(ub->ub_disk, false);
+       mutex_unlock(&ub->ub_disk->open_mutex);
+}
+
 static inline struct ublksrv_io_desc *
 ublk_get_iod(const struct ublk_queue *ubq, unsigned tag)
 {
@@ -2026,6 +2041,7 @@ static void ublk_stop_dev(struct ublk_device *ub)
        mutex_lock(&ub->mutex);
        ublk_stop_dev_unlocked(ub);
        mutex_unlock(&ub->mutex);
+       flush_work(&ub->partition_scan_work);
        ublk_cancel_dev(ub);
 }
 
@@ -2954,9 +2970,17 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub,
 
        ublk_apply_params(ub);
 
-       /* don't probe partitions if any daemon task is un-trusted */
-       if (ub->unprivileged_daemons)
-               set_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
+       /*
+        * Suppress partition scan to avoid potential IO hang.
+        *
+        * If ublk server error occurs during partition scan, the IO may
+        * wait while holding ub->mutex, which can deadlock with other
+        * operations that need the mutex. Defer partition scan to async
+        * work.
+        * For unprivileged daemons, keep GD_SUPPRESS_PART_SCAN set
+        * permanently.
+        */
+       set_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
 
        ublk_get_device(ub);
        ub->dev_info.state = UBLK_S_DEV_LIVE;
@@ -2973,6 +2997,10 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub,
 
        set_bit(UB_STATE_USED, &ub->state);
 
+       /* Schedule async partition scan for trusted daemons */
+       if (!ub->unprivileged_daemons)
+               schedule_work(&ub->partition_scan_work);
+
 out_put_cdev:
        if (ret) {
                ublk_detach_disk(ub);
@@ -3138,6 +3166,7 @@ static int ublk_ctrl_add_dev(const struct ublksrv_ctrl_cmd *header)
        mutex_init(&ub->mutex);
        spin_lock_init(&ub->lock);
        mutex_init(&ub->cancel_mutex);
+       INIT_WORK(&ub->partition_scan_work, ublk_partition_scan_work);
 
        ret = ublk_alloc_dev_number(ub, header->dev_id);
        if (ret < 0)