tcmu: fix unmap thread race

If the unmap thread has already run find_free_blocks
but not yet run prepare_to_wait when a wake_up(&unmap_wait)
call is done, the unmap thread is going to miss the wake
call. Instead of adding checks for if new waiters were added
this just has us use a work queue which will run us again
in this type of case.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_user.c

@@ -32,7 +32,7 @@
 #include <linux/highmem.h>
 #include <linux/configfs.h>
 #include <linux/mutex.h>
-#include <linux/kthread.h>
+#include <linux/workqueue.h>
 #include <net/genetlink.h>
 #include <scsi/scsi_common.h>
 #include <scsi/scsi_proto.h>
@@ -176,12 +176,11 @@ struct tcmu_cmd {
 	unsigned long flags;
 };
 
-static struct task_struct *unmap_thread;
-static wait_queue_head_t unmap_wait;
 static DEFINE_MUTEX(root_udev_mutex);
 static LIST_HEAD(root_udev);
 
 static atomic_t global_db_count = ATOMIC_INIT(0);
+static struct work_struct tcmu_unmap_work;
 
 static struct kmem_cache *tcmu_cmd_cache;
 
@@ -389,8 +388,7 @@ static bool tcmu_get_empty_blocks(struct tcmu_dev *udev,
 
 err:
 	udev->waiting_global = true;
-	/* Try to wake up the unmap thread */
-	wake_up(&unmap_wait);
+	schedule_work(&tcmu_unmap_work);
 	return false;
 }
 
@@ -1065,8 +1063,7 @@ static void tcmu_device_timedout(struct timer_list *t)
 	idr_for_each(&udev->commands, tcmu_check_expired_cmd, NULL);
 	spin_unlock_irqrestore(&udev->commands_lock, flags);
 
-	/* Try to wake up the ummap thread */
-	wake_up(&unmap_wait);
+	schedule_work(&tcmu_unmap_work);
 
 	/*
 	 * We don't need to wakeup threads on wait_cmdr since they have their
@@ -2044,23 +2041,10 @@ static void run_cmdr_queues(void)
 	mutex_unlock(&root_udev_mutex);
 }
 
-static int unmap_thread_fn(void *data)
+static void tcmu_unmap_work_fn(struct work_struct *work)
 {
-	while (!kthread_should_stop()) {
-		DEFINE_WAIT(__wait);
-
-		prepare_to_wait(&unmap_wait, &__wait, TASK_INTERRUPTIBLE);
-		schedule();
-		finish_wait(&unmap_wait, &__wait);
-
-		if (kthread_should_stop())
-			break;
-
-		find_free_blocks();
-		run_cmdr_queues();
-	}
-
-	return 0;
+	find_free_blocks();
+	run_cmdr_queues();
 }
 
 static int __init tcmu_module_init(void)
@@ -2069,6 +2053,8 @@ static int __init tcmu_module_init(void)
 
 	BUILD_BUG_ON((sizeof(struct tcmu_cmd_entry) % TCMU_OP_ALIGN_SIZE) != 0);
 
+	INIT_WORK(&tcmu_unmap_work, tcmu_unmap_work_fn);
+
 	tcmu_cmd_cache = kmem_cache_create("tcmu_cmd_cache",
 				sizeof(struct tcmu_cmd),
 				__alignof__(struct tcmu_cmd),
@@ -2114,17 +2100,8 @@ static int __init tcmu_module_init(void)
 	if (ret)
 		goto out_attrs;
 
-	init_waitqueue_head(&unmap_wait);
-	unmap_thread = kthread_run(unmap_thread_fn, NULL, "tcmu_unmap");
-	if (IS_ERR(unmap_thread)) {
-		ret = PTR_ERR(unmap_thread);
-		goto out_unreg_transport;
-	}
-
 	return 0;
 
-out_unreg_transport:
-	target_backend_unregister(&tcmu_ops);
 out_attrs:
 	kfree(tcmu_attrs);
 out_unreg_genl:
@@ -2139,7 +2116,7 @@ out_free_cache:
 
 static void __exit tcmu_module_exit(void)
 {
-	kthread_stop(unmap_thread);
+	cancel_work_sync(&tcmu_unmap_work);
 	target_backend_unregister(&tcmu_ops);
 	kfree(tcmu_attrs);
 	genl_unregister_family(&tcmu_genl_family);