[SCSI] Add ability to scan scsi busses asynchronously

Since it often takes around 20-30 seconds to scan a scsi bus, it's
highly advantageous to do this in parallel with other things. The bulk
of this patch is ensuring that devices don't change numbering, and that
all devices are discovered prior to trying to start init. For those
who build SCSI as modules, there's a new scsi_wait_scan module that will
ensure all bus scans are finished.

This patch only handles drivers which call scsi_scan_host. Fibre Channel,
SAS, SATA, USB and Firewire all need additional work.

Signed-off-by: Matthew Wilcox <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>

Documentation/kernel-parameters.txt

···
 
 	scsi_logging=	[SCSI]
 
+	scsi_mod.scan=	[SCSI] sync (default) scans SCSI busses as they are
+			discovered.  async scans them in kernel threads,
+			allowing boot to proceed.  none ignores them, expecting
+			user space to do the scan.
+
 	selinux		[SELINUX] Disable or enable SELinux at boot time.
 			Format: { "0" | "1" }
 			See security/selinux/Kconfig help text.

drivers/scsi/Makefile

···
 # This goes last, so that "real" scsi devices probe earlier
 obj-$(CONFIG_SCSI_DEBUG)	+= scsi_debug.o
 
+obj-$(CONFIG_SCSI)		+= scsi_wait_scan.o
+
 scsi_mod-y			+= scsi.o hosts.o scsi_ioctl.o constants.o \
 				   scsicam.o scsi_error.o scsi_lib.o \
 				   scsi_scan.o scsi_sysfs.o \

drivers/scsi/scsi_priv.h

···
 	{ };
 #endif
 
+/* scsi_scan.c */
+int scsi_complete_async_scans(void);
+
 /* scsi_devinfo.c */
 extern int scsi_get_device_flags(struct scsi_device *sdev,
 				 const unsigned char *vendor,

drivers/scsi/scsi_scan.c

···
 #include <linux/moduleparam.h>
 #include <linux/init.h>
 #include <linux/blkdev.h>
-#include <asm/semaphore.h>
+#include <linux/delay.h>
+#include <linux/kthread.h>
+#include <linux/spinlock.h>
 
 #include <scsi/scsi.h>
 #include <scsi/scsi_cmnd.h>
···
 MODULE_PARM_DESC(max_luns,
 		 "last scsi LUN (should be between 1 and 2^32-1)");
 
+static char scsi_scan_type[6] = "sync";
+
+module_param_string(scan, scsi_scan_type, sizeof(scsi_scan_type), S_IRUGO);
+MODULE_PARM_DESC(scan, "sync, async or none");
+
 /*
  * max_scsi_report_luns: the maximum number of LUNS that will be
  * returned from the REPORT LUNS command. 8 times this value must
···
 MODULE_PARM_DESC(inq_timeout, 
 		 "Timeout (in seconds) waiting for devices to answer INQUIRY."
 		 " Default is 5. Some non-compliant devices need more.");
+
+static DEFINE_SPINLOCK(async_scan_lock);
+static LIST_HEAD(scanning_hosts);
+
+struct async_scan_data {
+	struct list_head list;
+	struct Scsi_Host *shost;
+	struct completion prev_finished;
+};
+
+/**
+ * scsi_complete_async_scans - Wait for asynchronous scans to complete
+ *
+ * Asynchronous scans add themselves to the scanning_hosts list.  Once
+ * that list is empty, we know that the scans are complete.  Rather than
+ * waking up periodically to check the state of the list, we pretend to be
+ * a scanning task by adding ourselves at the end of the list and going to
+ * sleep.  When the task before us wakes us up, we take ourselves off the
+ * list and return.
+ */
+int scsi_complete_async_scans(void)
+{
+	struct async_scan_data *data;
+
+	do {
+		if (list_empty(&scanning_hosts))
+			return 0;
+		/* If we can't get memory immediately, that's OK.  Just
+		 * sleep a little.  Even if we never get memory, the async
+		 * scans will finish eventually.
+		 */
+		data = kmalloc(sizeof(*data), GFP_KERNEL);
+		if (!data)
+			msleep(1);
+	} while (!data);
+
+	data->shost = NULL;
+	init_completion(&data->prev_finished);
+
+	spin_lock(&async_scan_lock);
+	/* Check that there's still somebody else on the list */
+	if (list_empty(&scanning_hosts))
+		goto done;
+	list_add_tail(&data->list, &scanning_hosts);
+	spin_unlock(&async_scan_lock);
+
+	printk(KERN_INFO "scsi: waiting for bus probes to complete ...\n");
+	wait_for_completion(&data->prev_finished);
+
+	spin_lock(&async_scan_lock);
+	list_del(&data->list);
+ done:
+	spin_unlock(&async_scan_lock);
+
+	kfree(data);
+	return 0;
+}
+
+#ifdef MODULE
+/* Only exported for the benefit of scsi_wait_scan */
+EXPORT_SYMBOL_GPL(scsi_complete_async_scans);
+#endif
 
 /**
  * scsi_unlock_floptical - unlock device via a special MODE SENSE command
···
  *     SCSI_SCAN_LUN_PRESENT: a new scsi_device was allocated and initialized
  **/
 static int scsi_add_lun(struct scsi_device *sdev, unsigned char *inq_result,
-		int *bflags)
+		int *bflags, int async)
 {
 	/*
 	 * XXX do not save the inquiry, since it can change underneath us,
···
 	 * register it and tell the rest of the kernel
 	 * about it.
 	 */
-	if (scsi_sysfs_add_sdev(sdev) != 0)
+	if (!async && scsi_sysfs_add_sdev(sdev) != 0)
 		return SCSI_SCAN_NO_RESPONSE;
 
 	return SCSI_SCAN_LUN_PRESENT;
···
 		goto out_free_result;
 	}
 
-	res = scsi_add_lun(sdev, result, &bflags);
+	res = scsi_add_lun(sdev, result, &bflags, shost->async_scan);
 	if (res == SCSI_SCAN_LUN_PRESENT) {
 		if (bflags & BLIST_KEY) {
 			sdev->lockable = 0;
···
 {
 	struct Scsi_Host *shost = dev_to_shost(parent);
 
+	if (!shost->async_scan)
+		scsi_complete_async_scans();
+
 	mutex_lock(&shost->scan_mutex);
 	if (scsi_host_scan_allowed(shost))
 		__scsi_scan_target(parent, channel, id, lun, rescan);
···
 		"%s: <%u:%u:%u>\n",
 		__FUNCTION__, channel, id, lun));
 
+	if (!shost->async_scan)
+		scsi_complete_async_scans();
+
 	if (((channel != SCAN_WILD_CARD) && (channel > shost->max_channel)) ||
 	    ((id != SCAN_WILD_CARD) && (id >= shost->max_id)) ||
 	    ((lun != SCAN_WILD_CARD) && (lun > shost->max_lun)))
···
 	return 0;
 }
 
+static void scsi_sysfs_add_devices(struct Scsi_Host *shost)
+{
+	struct scsi_device *sdev;
+	shost_for_each_device(sdev, shost) {
+		if (scsi_sysfs_add_sdev(sdev) != 0)
+			scsi_destroy_sdev(sdev);
+	}
+}
+
+/**
+ * scsi_prep_async_scan - prepare for an async scan
+ * @shost: the host which will be scanned
+ * Returns: a cookie to be passed to scsi_finish_async_scan()
+ *
+ * Tells the midlayer this host is going to do an asynchronous scan.
+ * It reserves the host's position in the scanning list and ensures
+ * that other asynchronous scans started after this one won't affect the
+ * ordering of the discovered devices.
+ */
+struct async_scan_data *scsi_prep_async_scan(struct Scsi_Host *shost)
+{
+	struct async_scan_data *data;
+
+	if (strncmp(scsi_scan_type, "sync", 4) == 0)
+		return NULL;
+
+	if (shost->async_scan) {
+		printk("%s called twice for host %d", __FUNCTION__,
+				shost->host_no);
+		dump_stack();
+		return NULL;
+	}
+
+	data = kmalloc(sizeof(*data), GFP_KERNEL);
+	if (!data)
+		goto err;
+	data->shost = scsi_host_get(shost);
+	if (!data->shost)
+		goto err;
+	init_completion(&data->prev_finished);
+
+	spin_lock(&async_scan_lock);
+	shost->async_scan = 1;
+	if (list_empty(&scanning_hosts))
+		complete(&data->prev_finished);
+	list_add_tail(&data->list, &scanning_hosts);
+	spin_unlock(&async_scan_lock);
+
+	return data;
+
+ err:
+	kfree(data);
+	return NULL;
+}
+
+/**
+ * scsi_finish_async_scan - asynchronous scan has finished
+ * @data: cookie returned from earlier call to scsi_prep_async_scan()
+ *
+ * All the devices currently attached to this host have been found.
+ * This function announces all the devices it has found to the rest
+ * of the system.
+ */
+void scsi_finish_async_scan(struct async_scan_data *data)
+{
+	struct Scsi_Host *shost;
+
+	if (!data)
+		return;
+
+	shost = data->shost;
+	if (!shost->async_scan) {
+		printk("%s called twice for host %d", __FUNCTION__,
+				shost->host_no);
+		dump_stack();
+		return;
+	}
+
+	wait_for_completion(&data->prev_finished);
+
+	scsi_sysfs_add_devices(shost);
+
+	spin_lock(&async_scan_lock);
+	shost->async_scan = 0;
+	list_del(&data->list);
+	if (!list_empty(&scanning_hosts)) {
+		struct async_scan_data *next = list_entry(scanning_hosts.next,
+				struct async_scan_data, list);
+		complete(&next->prev_finished);
+	}
+	spin_unlock(&async_scan_lock);
+
+	scsi_host_put(shost);
+	kfree(data);
+}
+
+static int do_scan_async(void *_data)
+{
+	struct async_scan_data *data = _data;
+	scsi_scan_host_selected(data->shost, SCAN_WILD_CARD, SCAN_WILD_CARD,
+				SCAN_WILD_CARD, 0);
+
+	scsi_finish_async_scan(data);
+	return 0;
+}
+
 /**
  * scsi_scan_host - scan the given adapter
  * @shost:	adapter to scan
  **/
 void scsi_scan_host(struct Scsi_Host *shost)
 {
-	scsi_scan_host_selected(shost, SCAN_WILD_CARD, SCAN_WILD_CARD,
-				SCAN_WILD_CARD, 0);
+	struct async_scan_data *data;
+
+	if (strncmp(scsi_scan_type, "none", 4) == 0)
+		return;
+
+	data = scsi_prep_async_scan(shost);
+	if (!data) {
+		scsi_scan_host_selected(shost, SCAN_WILD_CARD, SCAN_WILD_CARD,
+					SCAN_WILD_CARD, 0);
+		return;
+	}
+	kthread_run(do_scan_async, data, "scsi_scan_%d", shost->host_no);
 }
 EXPORT_SYMBOL(scsi_scan_host);
 

drivers/scsi/scsi_wait_scan.c

···
+/*
+ * scsi_wait_scan.c
+ *
+ * Copyright (C) 2006 James Bottomley <[e-mail]>
+ *
+ * This is a simple module to wait until all the async scans are
+ * complete.  The idea is to use it in initrd/initramfs scripts.  You
+ * modprobe it after all the modprobes of the root SCSI drivers and it
+ * will wait until they have all finished scanning their busses before
+ * allowing the boot to proceed
+ */
+
+#include <linux/module.h>
+#include "scsi_priv.h"
+
+static int __init wait_scan_init(void)
+{
+	scsi_complete_async_scans();
+	return 0;
+}
+
+static void __exit wait_scan_exit(void)
+{
+}
+
+MODULE_DESCRIPTION("SCSI wait for scans");
+MODULE_AUTHOR("James Bottomley");
+MODULE_LICENSE("GPL");
+
+late_initcall(wait_scan_init);
+module_exit(wait_scan_exit);

include/scsi/scsi_device.h

···
 						  struct scsi_device *);
 
 /**
- * shost_for_each_device  -  iterate over all devices of a host
- * @sdev:	iterator
- * @host:	host whiches devices we want to iterate over
+ * shost_for_each_device - iterate over all devices of a host
+ * @sdev: the &struct scsi_device to use as a cursor
+ * @shost: the &struct scsi_host to iterate over
  *
- * This traverses over each devices of @shost.  The devices have
- * a reference that must be released by scsi_host_put when breaking
- * out of the loop.
+ * Iterator that returns each device attached to @shost.  This loop
+ * takes a reference on each device and releases it at the end.  If
+ * you break out of the loop, you must call scsi_device_put(sdev).
  */
 #define shost_for_each_device(sdev, shost) \
 	for ((sdev) = __scsi_iterate_devices((shost), NULL); \
···
 	     (sdev) = __scsi_iterate_devices((shost), (sdev)))
 
 /**
- * __shost_for_each_device  -  iterate over all devices of a host (UNLOCKED)
- * @sdev:	iterator
- * @host:	host whiches devices we want to iterate over
+ * __shost_for_each_device - iterate over all devices of a host (UNLOCKED)
+ * @sdev: the &struct scsi_device to use as a cursor
+ * @shost: the &struct scsi_host to iterate over
  *
- * This traverses over each devices of @shost.  It does _not_ take a
- * reference on the scsi_device, thus it the whole loop must be protected
- * by shost->host_lock.
+ * Iterator that returns each device attached to @shost.  It does _not_
+ * take a reference on the scsi_device, so the whole loop must be
+ * protected by shost->host_lock.
  *
- * Note:  The only reason why drivers would want to use this is because
- * they're need to access the device list in irq context.  Otherwise you
- * really want to use shost_for_each_device instead.
+ * Note: The only reason to use this is because you need to access the
+ * device list in interrupt context.  Otherwise you really want to use
+ * shost_for_each_device instead.
  */
 #define __shost_for_each_device(sdev, shost) \
 	list_for_each_entry((sdev), &((shost)->__devices), siblings)

include/scsi/scsi_host.h

···
 	/* task mgmt function in progress */
 	unsigned tmf_in_progress:1;
 
+	/* Asynchronous scan in progress */
+	unsigned async_scan:1;
+
 	/*
 	 * Optional work queue to be utilized by the transport
 	 */