scsi: Rework handling of scsi_device.vpd_pg8[03]

Introduce struct scsi_vpd for the VPD page length, data and the RCU head
that will be used to free the VPD data. Use kfree_rcu() instead of
kfree() to free VPD data. Move the VPD buffer pointer check inside the
RCU read lock in the sysfs code. Only annotate pointers that are shared
across threads with __rcu. Use rcu_dereference() when dereferencing an
RCU pointer. This patch suppresses about twenty sparse complaints about
the vpd_pg8[03] pointers. This patch also fixes a race condition, namely
that updating of the VPD pointers and length variables in struct
scsi_device was not atomic with reference to the code reading these
variables. See also "Does the update code tolerate concurrent accesses?"
in Documentation/RCU/checklist.txt.

Fixes: commit 09e2b0b14690 ("scsi: rescan VPD attributes")
Signed-off-by: Bart Van Assche <[e-mail]>
Acked-by: Hannes Reinecke <[e-mail]>
Reviewed-by: Shane Seymour <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Johannes Thumshirn <[e-mail]>
Cc: Shane Seymour <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/scsi.c

···
  * scsi_get_vpd_buf - Get Vital Product Data from a SCSI device
  * @sdev: The device to ask
  * @page: Which Vital Product Data to return
- * @len: Upon success, the VPD length will be stored in *@len.
  *
  * Returns %NULL upon failure.
  */
-static unsigned char *scsi_get_vpd_buf(struct scsi_device *sdev, u8 page,
-				       int *len)
+static struct scsi_vpd *scsi_get_vpd_buf(struct scsi_device *sdev, u8 page)
 {
-	unsigned char *vpd_buf;
+	struct scsi_vpd *vpd_buf;
 	int vpd_len = SCSI_VPD_PG_LEN, result;
 
 retry_pg:
-	vpd_buf = kmalloc(vpd_len, GFP_KERNEL);
+	vpd_buf = kmalloc(sizeof(*vpd_buf) + vpd_len, GFP_KERNEL);
 	if (!vpd_buf)
 		return NULL;
 
-	result = scsi_vpd_inquiry(sdev, vpd_buf, page, vpd_len);
+	result = scsi_vpd_inquiry(sdev, vpd_buf->data, page, vpd_len);
 	if (result < 0) {
 		kfree(vpd_buf);
 		return NULL;
···
 		goto retry_pg;
 	}
 
-	*len = result;
+	vpd_buf->len = result;
 
 	return vpd_buf;
 }
 
 static void scsi_update_vpd_page(struct scsi_device *sdev, u8 page,
-				 unsigned char __rcu **sdev_vpd_buf,
-				 int *sdev_vpd_len)
+				 struct scsi_vpd __rcu **sdev_vpd_buf)
 {
-	unsigned char *vpd_buf;
-	int len;
+	struct scsi_vpd *vpd_buf;
 
-	vpd_buf = scsi_get_vpd_buf(sdev, page, &len);
+	vpd_buf = scsi_get_vpd_buf(sdev, page);
 	if (!vpd_buf)
 		return;
 
 	mutex_lock(&sdev->inquiry_mutex);
 	rcu_swap_protected(*sdev_vpd_buf, vpd_buf,
 			   lockdep_is_held(&sdev->inquiry_mutex));
-	*sdev_vpd_len = len;
 	mutex_unlock(&sdev->inquiry_mutex);
 
-	synchronize_rcu();
-
-	kfree(vpd_buf);
+	if (vpd_buf)
+		kfree_rcu(vpd_buf, rcu);
 }
 
 /**
···
  */
 void scsi_attach_vpd(struct scsi_device *sdev)
 {
-	int i, vpd_len;
-	unsigned char *vpd_buf;
+	int i;
+	struct scsi_vpd *vpd_buf;
 
 	if (!scsi_device_supports_vpd(sdev))
 		return;
 
 	/* Ask for all the pages supported by this device */
-	vpd_buf = scsi_get_vpd_buf(sdev, 0, &vpd_len);
+	vpd_buf = scsi_get_vpd_buf(sdev, 0);
 	if (!vpd_buf)
 		return;
 
-	for (i = 4; i < vpd_len; i++) {
-		if (vpd_buf[i] == 0x80)
-			scsi_update_vpd_page(sdev, 0x80, &sdev->vpd_pg80,
-					     &sdev->vpd_pg80_len);
-		if (vpd_buf[i] == 0x83)
-			scsi_update_vpd_page(sdev, 0x83, &sdev->vpd_pg83,
-					     &sdev->vpd_pg83_len);
+	for (i = 4; i < vpd_buf->len; i++) {
+		if (vpd_buf->data[i] == 0x80)
+			scsi_update_vpd_page(sdev, 0x80, &sdev->vpd_pg80);
+		if (vpd_buf->data[i] == 0x83)
+			scsi_update_vpd_page(sdev, 0x83, &sdev->vpd_pg83);
 	}
 	kfree(vpd_buf);
 }

drivers/scsi/scsi_lib.c

···
 {
 	u8 cur_id_type = 0xff;
 	u8 cur_id_size = 0;
-	unsigned char *d, *cur_id_str;
-	unsigned char __rcu *vpd_pg83;
+	const unsigned char *d, *cur_id_str;
+	const struct scsi_vpd *vpd_pg83;
 	int id_size = -EINVAL;
 
 	rcu_read_lock();
···
 	}
 
 	memset(id, 0, id_len);
-	d = vpd_pg83 + 4;
-	while (d < vpd_pg83 + sdev->vpd_pg83_len) {
+	d = vpd_pg83->data + 4;
+	while (d < vpd_pg83->data + vpd_pg83->len) {
 		/* Skip designators not referring to the LUN */
 		if ((d[1] & 0x30) != 0x00)
 			goto next_desig;
···
  */
 int scsi_vpd_tpg_id(struct scsi_device *sdev, int *rel_id)
 {
-	unsigned char *d;
-	unsigned char __rcu *vpd_pg83;
+	const unsigned char *d;
+	const struct scsi_vpd *vpd_pg83;
 	int group_id = -EAGAIN, rel_port = -1;
 
 	rcu_read_lock();
···
 		return -ENXIO;
 	}
 
-	d = sdev->vpd_pg83 + 4;
-	while (d < sdev->vpd_pg83 + sdev->vpd_pg83_len) {
+	d = vpd_pg83->data + 4;
+	while (d < vpd_pg83->data + vpd_pg83->len) {
 		switch (d[1] & 0xf) {
 		case 0x4:
 			/* Relative target port */

drivers/scsi/scsi_sysfs.c

···
 	struct scsi_device *sdev;
 	struct device *parent;
 	struct list_head *this, *tmp;
+	struct scsi_vpd *vpd_pg80 = NULL, *vpd_pg83 = NULL;
 	unsigned long flags;
 
 	sdev = container_of(work, struct scsi_device, ew.work);
···
 	/* NULL queue means the device can't be used */
 	sdev->request_queue = NULL;
 
-	kfree(sdev->vpd_pg83);
-	kfree(sdev->vpd_pg80);
+	mutex_lock(&sdev->inquiry_mutex);
+	rcu_swap_protected(sdev->vpd_pg80, vpd_pg80,
+			   lockdep_is_held(&sdev->inquiry_mutex));
+	rcu_swap_protected(sdev->vpd_pg83, vpd_pg83,
+			   lockdep_is_held(&sdev->inquiry_mutex));
+	mutex_unlock(&sdev->inquiry_mutex);
+
+	if (vpd_pg83)
+		kfree_rcu(vpd_pg83, rcu);
+	if (vpd_pg80)
+		kfree_rcu(vpd_pg80, rcu);
 	kfree(sdev->inquiry);
 	kfree(sdev);
 
···
 {									\
 	struct device *dev = container_of(kobj, struct device, kobj);	\
 	struct scsi_device *sdev = to_scsi_device(dev);			\
-	int ret;							\
-	if (!sdev->vpd_##_page)						\
-		return -EINVAL;						\
+	struct scsi_vpd *vpd_page;					\
+	int ret = -EINVAL;						\
+									\
 	rcu_read_lock();						\
-	ret = memory_read_from_buffer(buf, count, &off,			\
-				      rcu_dereference(sdev->vpd_##_page), \
-				       sdev->vpd_##_page##_len);	\
+	vpd_page = rcu_dereference(sdev->vpd_##_page);			\
+	if (vpd_page)							\
+		ret = memory_read_from_buffer(buf, count, &off,		\
+				vpd_page->data, vpd_page->len);		\
 	rcu_read_unlock();						\
-	return ret;						\
+	return ret;							\
 }									\
 static struct bin_attribute dev_attr_vpd_##_page = {		\
 	.attr =	{.name = __stringify(vpd_##_page), .mode = S_IRUGO },	\

include/scsi/scsi_device.h

···
 	 */
 };
 
+/**
+ * struct scsi_vpd - SCSI Vital Product Data
+ * @rcu: For kfree_rcu().
+ * @len: Length in bytes of @data.
+ * @data: VPD data as defined in various T10 SCSI standard documents.
+ */
+struct scsi_vpd {
+	struct rcu_head	rcu;
+	int		len;
+	unsigned char	data[];
+};
+
 struct scsi_device {
 	struct Scsi_Host *host;
 	struct request_queue *request_queue;
···
 	const char * rev;		/* ... "nullnullnullnull" before scan */
 
 #define SCSI_VPD_PG_LEN                255
-	int vpd_pg83_len;
-	unsigned char __rcu *vpd_pg83;
-	int vpd_pg80_len;
-	unsigned char __rcu *vpd_pg80;
+	struct scsi_vpd __rcu *vpd_pg83;
+	struct scsi_vpd __rcu *vpd_pg80;
 	unsigned char current_tag;	/* current tag */
 	struct scsi_target      *sdev_target;   /* used only for single_lun */