[SCSI] export command allocation and freeing functions independently of the host

This is needed by things like USB storage that want to set up static
commands for later use at start of day.
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/scsi.c

@@ -330,30 +330,16 @@ void scsi_put_command(struct scsi_cmnd *cmd)
 }
 EXPORT_SYMBOL(scsi_put_command);
 
-/**
- * scsi_setup_command_freelist - Setup the command freelist for a scsi host.
- * @shost: host to allocate the freelist for.
- *
- * Description: The command freelist protects against system-wide out of memory
- * deadlock by preallocating one SCSI command structure for each host, so the
- * system can always write to a swap file on a device associated with that host.
- *
- * Returns:	Nothing.
- */
-int scsi_setup_command_freelist(struct Scsi_Host *shost)
+static struct scsi_host_cmd_pool *scsi_get_host_cmd_pool(gfp_t gfp_mask)
 {
-	struct scsi_host_cmd_pool *pool;
-	struct scsi_cmnd *cmd;
-
-	spin_lock_init(&shost->free_list_lock);
-	INIT_LIST_HEAD(&shost->free_list);
-
+	struct scsi_host_cmd_pool *retval = NULL, *pool;
 	/*
 	 * Select a command slab for this host and create it if not
 	 * yet existent.
 	 */
 	mutex_lock(&host_cmd_pool_mutex);
-	pool = (shost->unchecked_isa_dma ? &scsi_cmd_dma_pool : &scsi_cmd_pool);
+	pool = (gfp_mask & __GFP_DMA) ? &scsi_cmd_dma_pool :
+		&scsi_cmd_pool;
 	if (!pool->users) {
 		pool->cmd_slab = kmem_cache_create(pool->cmd_name,
 						   sizeof(struct scsi_cmnd), 0,
@@ -371,28 +357,122 @@ int scsi_setup_command_freelist(struct Scsi_Host *shost)
 	}
 
 	pool->users++;
-	shost->cmd_pool = pool;
+	retval = pool;
+ fail:
 	mutex_unlock(&host_cmd_pool_mutex);
+	return retval;
+}
+
+static void scsi_put_host_cmd_pool(gfp_t gfp_mask)
+{
+	struct scsi_host_cmd_pool *pool;
 
+	mutex_lock(&host_cmd_pool_mutex);
+	pool = (gfp_mask & __GFP_DMA) ? &scsi_cmd_dma_pool :
+		&scsi_cmd_pool;
 	/*
-	 * Get one backup command for this host.
+	 * This may happen if a driver has a mismatched get and put
+	 * of the command pool; the driver should be implicated in
+	 * the stack trace
 	 */
-	cmd = scsi_pool_alloc_command(shost->cmd_pool, GFP_KERNEL);
-	if (!cmd)
-		goto fail2;
+	BUG_ON(pool->users == 0);
 
-	list_add(&cmd->list, &shost->free_list);
-	return 0;
-
- fail2:
-	mutex_lock(&host_cmd_pool_mutex);
 	if (!--pool->users) {
 		kmem_cache_destroy(pool->cmd_slab);
 		kmem_cache_destroy(pool->sense_slab);
 	}
- fail:
 	mutex_unlock(&host_cmd_pool_mutex);
-	return -ENOMEM;
+}
+
+/**
+ * scsi_allocate_command - get a fully allocated SCSI command
+ * @gfp_mask:	allocation mask
+ *
+ * This function is for use outside of the normal host based pools.
+ * It allocates the relevant command and takes an additional reference
+ * on the pool it used.  This function *must* be paired with
+ * scsi_free_command which also has the identical mask, otherwise the
+ * free pool counts will eventually go wrong and you'll trigger a bug.
+ *
+ * This function should *only* be used by drivers that need a static
+ * command allocation at start of day for internal functions.
+ */
+struct scsi_cmnd *scsi_allocate_command(gfp_t gfp_mask)
+{
+	struct scsi_host_cmd_pool *pool = scsi_get_host_cmd_pool(gfp_mask);
+
+	if (!pool)
+		return NULL;
+
+	return scsi_pool_alloc_command(pool, gfp_mask);
+}
+EXPORT_SYMBOL(scsi_allocate_command);
+
+/**
+ * scsi_free_command - free a command allocated by scsi_allocate_command
+ * @gfp_mask:	mask used in the original allocation
+ * @cmd:	command to free
+ *
+ * Note: using the original allocation mask is vital because that's
+ * what determines which command pool we use to free the command.  Any
+ * mismatch will cause the system to BUG eventually.
+ */
+void scsi_free_command(gfp_t gfp_mask, struct scsi_cmnd *cmd)
+{
+	struct scsi_host_cmd_pool *pool = scsi_get_host_cmd_pool(gfp_mask);
+
+	/*
+	 * this could trigger if the mask to scsi_allocate_command
+	 * doesn't match this mask.  Otherwise we're guaranteed that this
+	 * succeeds because scsi_allocate_command must have taken a reference
+	 * on the pool
+	 */
+	BUG_ON(!pool);
+
+	scsi_pool_free_command(pool, cmd);
+	/*
+	 * scsi_put_host_cmd_pool is called twice; once to release the
+	 * reference we took above, and once to release the reference
+	 * originally taken by scsi_allocate_command
+	 */
+	scsi_put_host_cmd_pool(gfp_mask);
+	scsi_put_host_cmd_pool(gfp_mask);
+}
+EXPORT_SYMBOL(scsi_free_command);
+
+/**
+ * scsi_setup_command_freelist - Setup the command freelist for a scsi host.
+ * @shost: host to allocate the freelist for.
+ *
+ * Description: The command freelist protects against system-wide out of memory
+ * deadlock by preallocating one SCSI command structure for each host, so the
+ * system can always write to a swap file on a device associated with that host.
+ *
+ * Returns:	Nothing.
+ */
+int scsi_setup_command_freelist(struct Scsi_Host *shost)
+{
+	struct scsi_cmnd *cmd;
+	const gfp_t gfp_mask = shost->unchecked_isa_dma ? GFP_DMA : GFP_KERNEL;
+
+	spin_lock_init(&shost->free_list_lock);
+	INIT_LIST_HEAD(&shost->free_list);
+
+	shost->cmd_pool = scsi_get_host_cmd_pool(gfp_mask);
+
+	if (!shost->cmd_pool)
+		return -ENOMEM;
+
+	/*
+	 * Get one backup command for this host.
+	 */
+	cmd = scsi_pool_alloc_command(shost->cmd_pool, gfp_mask);
+	if (!cmd) {
+		scsi_put_host_cmd_pool(gfp_mask);
+		return -ENOMEM;
+	}
+	list_add(&cmd->list, &shost->free_list);
+	return 0;
 }
 
 /**
@@ -408,13 +488,8 @@ void scsi_destroy_command_freelist(struct Scsi_Host *shost)
 		list_del_init(&cmd->list);
 		scsi_pool_free_command(shost->cmd_pool, cmd);
 	}
-
-	mutex_lock(&host_cmd_pool_mutex);
-	if (!--shost->cmd_pool->users) {
-		kmem_cache_destroy(shost->cmd_pool->cmd_slab);
-		kmem_cache_destroy(shost->cmd_pool->sense_slab);
-	}
-	mutex_unlock(&host_cmd_pool_mutex);
+	shost->cmd_pool = NULL;
+	scsi_put_host_cmd_pool(shost->unchecked_isa_dma ? GFP_DMA : GFP_KERNEL);
 }
 
 #ifdef CONFIG_SCSI_LOGGING

include/scsi/scsi_cmnd.h

@@ -130,6 +130,9 @@ extern void scsi_release_buffers(struct scsi_cmnd *cmd);
 extern int scsi_dma_map(struct scsi_cmnd *cmd);
 extern void scsi_dma_unmap(struct scsi_cmnd *cmd);
 
+struct scsi_cmnd *scsi_allocate_command(gfp_t gfp_mask);
+void scsi_free_command(gfp_t gfp_mask, struct scsi_cmnd *cmd);
+
 static inline unsigned scsi_sg_count(struct scsi_cmnd *cmd)
 {
 	return cmd->sdb.table.nents;