scsi: sd: Rely on the driver core for asynchronous probing

As explained during the 2018 LSF/MM session about increasing SCSI disk
probing concurrency, the problems with the current probing approach are as
follows:

 - The driver core is unaware of asynchronous SCSI LUN probing.
   wait_for_device_probe() waits for all asynchronous probes except
   asynchronous SCSI disk probes.

 - There is unnecessary serialization between sd_probe() and sd_remove().
   This can lead to a deadlock.

Hence this patch that modifies the sd driver such that it uses the driver
core framework for asynchronous probing. The async domain and
get_device()/put_device() pairs that became superfluous due to this change
are removed.

This patch does not affect the time needed for loading the scsi_debug
kernel module with parameters delay=0 and max_luns=256.

This patch depends on commit ef0ff683 ("driver core: Probe devices
asynchronously instead of the driver") that went upstream in kernel version
v5.1-rc1.

Cc: Lee Duncan <lduncan@suse.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi.c

@@ -86,15 +86,10 @@ unsigned int scsi_logging_level;
 EXPORT_SYMBOL(scsi_logging_level);
 #endif
 
-/* sd, scsi core and power management need to coordinate flushing async actions */
-ASYNC_DOMAIN(scsi_sd_probe_domain);
-EXPORT_SYMBOL(scsi_sd_probe_domain);
-
 /*
- * Separate domain (from scsi_sd_probe_domain) to maximize the benefit of
- * asynchronous system resume operations.  It is marked 'exclusive' to avoid
- * being included in the async_synchronize_full() that is invoked by
- * dpm_resume()
+ * Domain for asynchronous system resume operations.  It is marked 'exclusive'
+ * to avoid being included in the async_synchronize_full() that is invoked by
+ * dpm_resume().
  */
 ASYNC_DOMAIN_EXCLUSIVE(scsi_sd_pm_domain);
 EXPORT_SYMBOL(scsi_sd_pm_domain);
@@ -821,7 +816,6 @@ static void __exit exit_scsi(void)
 	scsi_exit_devinfo();
 	scsi_exit_procfs();
 	scsi_exit_queue();
-	async_unregister_domain(&scsi_sd_probe_domain);
 }
 
 subsys_initcall(init_scsi);

drivers/scsi/scsi_pm.c

@@ -176,11 +176,7 @@ static int scsi_bus_resume_common(struct device *dev,
 
 static int scsi_bus_prepare(struct device *dev)
 {
-	if (scsi_is_sdev_device(dev)) {
-		/* sd probing uses async_schedule.  Wait until it finishes. */
-		async_synchronize_full_domain(&scsi_sd_probe_domain);
-
-	} else if (scsi_is_host_device(dev)) {
+	if (scsi_is_host_device(dev)) {
 		/* Wait until async scanning is finished */
 		scsi_complete_async_scans();
 	}

drivers/scsi/scsi_priv.h

@@ -175,7 +175,6 @@ static inline void scsi_autopm_put_host(struct Scsi_Host *h) {}
 #endif /* CONFIG_PM */
 
 extern struct async_domain scsi_sd_pm_domain;
-extern struct async_domain scsi_sd_probe_domain;
 
 /* scsi_dh.c */
 #ifdef CONFIG_SCSI_DH

drivers/scsi/sd.c

@@ -568,6 +568,7 @@ static struct scsi_driver sd_template = {
 		.name		= "sd",
 		.owner		= THIS_MODULE,
 		.probe		= sd_probe,
+		.probe_type	= PROBE_PREFER_ASYNCHRONOUS,
 		.remove		= sd_remove,
 		.shutdown	= sd_shutdown,
 		.pm		= &sd_pm_ops,
@@ -3253,12 +3254,8 @@ static int sd_format_disk_name(char *prefix, int index, char *buf, int buflen)
 	return 0;
 }
 
-/*
- * The asynchronous part of sd_probe
- */
-static void sd_probe_async(void *data, async_cookie_t cookie)
+static void sd_probe_part2(struct scsi_disk *sdkp)
 {
-	struct scsi_disk *sdkp = data;
 	struct scsi_device *sdp;
 	struct gendisk *gd;
 	u32 index;
@@ -3313,7 +3310,6 @@ static void sd_probe_async(void *data, async_cookie_t cookie)
 	sd_printk(KERN_NOTICE, sdkp, "Attached SCSI %sdisk\n",
 		  sdp->removable ? "removable " : "");
 	scsi_autopm_put_device(sdp);
-	put_device(&sdkp->dev);
 }
 
 /**
@@ -3405,8 +3401,7 @@ static int sd_probe(struct device *dev)
 	get_device(dev);
 	dev_set_drvdata(dev, sdkp);
 
-	get_device(&sdkp->dev);	/* prevent release before async_schedule */
-	async_schedule_domain(sd_probe_async, sdkp, &scsi_sd_probe_domain);
+	sd_probe_part2(sdkp);
 
 	return 0;
 
@@ -3442,7 +3437,6 @@ static int sd_remove(struct device *dev)
 	scsi_autopm_get_device(sdkp->device);
 
 	async_synchronize_full_domain(&scsi_sd_pm_domain);
-	async_synchronize_full_domain(&scsi_sd_probe_domain);
 	device_del(&sdkp->dev);
 	del_gendisk(sdkp->disk);
 	sd_shutdown(dev);