From: Dongxiao Xu <dongxiao.xu@intel.com>
Subject: [PATCH 3/3] Use Kernel thread to replace the tasklet.
Patch-mainline: n/a

 Kernel thread has more control over QoS, and could improve
 dom0's userspace responseness.

Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>

Subject: xen: ensure locking gnttab_copy_grant_page is safe against interrupts.

Now that netback processing occurs in a thread instead of a tasklet
gnttab_copy_grant_page needs to be safe against interrupts.

The code is currently commented out in this tree but on 2.6.18 we observed a
deadlock where the netback thread called gnttab_copy_grant_page, locked
gnttab_dma_lock for writing, was interrupted and on return from interrupt the
network stack's TX tasklet ended up calling __gnttab_dma_map_page via the
hardware driver->swiotlb and tries to take gnttab_dma_lock for reading.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>#
Cc: "Xu, Dongxiao" <dongxiao.xu@intel.com>

jb: changed write_seq{,un}lock_irq() to write_seq{,un}lock_bh(), and
    made the use of kernel threads optional (but default)
Acked-by: jbeulich@novell.com

--- head-2010-04-29.orig/drivers/xen/core/gnttab.c	2010-04-15 11:42:34.000000000 +0200
+++ head-2010-04-29/drivers/xen/core/gnttab.c	2010-04-15 11:44:26.000000000 +0200
@@ -554,14 +554,14 @@ int gnttab_copy_grant_page(grant_ref_t r
 	mfn = pfn_to_mfn(pfn);
 	new_mfn = virt_to_mfn(new_addr);
 
-	write_seqlock(&gnttab_dma_lock);
+	write_seqlock_bh(&gnttab_dma_lock);
 
 	/* Make seq visible before checking page_mapped. */
 	smp_mb();
 
 	/* Has the page been DMA-mapped? */
 	if (unlikely(page_mapped(page))) {
-		write_sequnlock(&gnttab_dma_lock);
+		write_sequnlock_bh(&gnttab_dma_lock);
 		put_page(new_page);
 		err = -EBUSY;
 		goto out;
@@ -578,7 +578,7 @@ int gnttab_copy_grant_page(grant_ref_t r
 	BUG_ON(err);
 	BUG_ON(unmap.status);
 
-	write_sequnlock(&gnttab_dma_lock);
+	write_sequnlock_bh(&gnttab_dma_lock);
 
 	if (!xen_feature(XENFEAT_auto_translated_physmap)) {
 		set_phys_to_machine(page_to_pfn(new_page), INVALID_P2M_ENTRY);
--- head-2010-04-29.orig/drivers/xen/netback/common.h	2010-04-30 11:32:08.000000000 +0200
+++ head-2010-04-29/drivers/xen/netback/common.h	2010-04-30 11:32:26.000000000 +0200
@@ -239,8 +239,16 @@ struct netbk_tx_pending_inuse {
 #define MAX_MFN_ALLOC 64
 
 struct xen_netbk {
-	struct tasklet_struct net_tx_tasklet;
-	struct tasklet_struct net_rx_tasklet;
+	union {
+		struct {
+			struct tasklet_struct net_tx_tasklet;
+			struct tasklet_struct net_rx_tasklet;
+		};
+		struct {
+			wait_queue_head_t netbk_action_wq;
+			struct task_struct *task;
+		};
+	};
 
 	struct sk_buff_head rx_queue;
 	struct sk_buff_head tx_queue;
--- head-2010-04-29.orig/drivers/xen/netback/netback.c	2010-04-30 11:49:27.000000000 +0200
+++ head-2010-04-29/drivers/xen/netback/netback.c	2010-04-30 11:49:32.000000000 +0200
@@ -35,6 +35,7 @@
  */
 
 #include "common.h"
+#include <linux/kthread.h>
 #include <linux/vmalloc.h>
 #include <xen/balloon.h>
 #include <xen/interface/memory.h>
@@ -43,6 +44,8 @@
 
 struct xen_netbk *__read_mostly xen_netbk;
 unsigned int __read_mostly netbk_nr_groups;
+static bool __read_mostly use_kthreads = true;
+static bool __initdata bind_threads;
 
 #define GET_GROUP_INDEX(netif) ((netif)->group)
 
@@ -127,7 +130,11 @@ static int MODPARM_permute_returns = 0;
 module_param_named(permute_returns, MODPARM_permute_returns, bool, S_IRUSR|S_IWUSR);
 MODULE_PARM_DESC(permute_returns, "Randomly permute the order in which TX responses are sent to the frontend");
 module_param_named(groups, netbk_nr_groups, uint, 0);
-MODULE_PARM_DESC(groups, "Specify the number of tasklet pairs to use");
+MODULE_PARM_DESC(groups, "Specify the number of tasklet pairs/threads to use");
+module_param_named(tasklets, use_kthreads, invbool, 0);
+MODULE_PARM_DESC(tasklets, "Use tasklets instead of kernel threads");
+module_param_named(bind, bind_threads, bool, 0);
+MODULE_PARM_DESC(bind, "Bind kernel threads to (v)CPUs");
 
 int netbk_copy_skb_mode;
 
@@ -164,8 +171,12 @@ static inline void maybe_schedule_tx_act
 
 	smp_mb();
 	if ((nr_pending_reqs(netbk) < (MAX_PENDING_REQS/2)) &&
-	    !list_empty(&netbk->net_schedule_list))
-		tasklet_schedule(&netbk->net_tx_tasklet);
+	    !list_empty(&netbk->net_schedule_list)) {
+		if (use_kthreads)
+			wake_up(&netbk->netbk_action_wq);
+		else
+			tasklet_schedule(&netbk->net_tx_tasklet);
+	}
 }
 
 static struct sk_buff *netbk_copy_skb(struct sk_buff *skb)
@@ -326,7 +337,10 @@ int netif_be_start_xmit(struct sk_buff *
 
 	netbk = &xen_netbk[GET_GROUP_INDEX(netif)];
 	skb_queue_tail(&netbk->rx_queue, skb);
-	tasklet_schedule(&netbk->net_rx_tasklet);
+	if (use_kthreads)
+		wake_up(&netbk->netbk_action_wq);
+	else
+		tasklet_schedule(&netbk->net_rx_tasklet);
 
 	return NETDEV_TX_OK;
 
@@ -779,8 +793,12 @@ static void net_rx_action(unsigned long 
 
 	/* More work to do? */
 	if (!skb_queue_empty(&netbk->rx_queue) &&
-	    !timer_pending(&netbk->net_timer))
-		tasklet_schedule(&netbk->net_rx_tasklet);
+	    !timer_pending(&netbk->net_timer)) {
+		if (use_kthreads)
+			wake_up(&netbk->netbk_action_wq);
+		else
+			tasklet_schedule(&netbk->net_rx_tasklet);
+	}
 #if 0
 	else
 		xen_network_done_notify();
@@ -789,12 +807,18 @@ static void net_rx_action(unsigned long 
 
 static void net_alarm(unsigned long group)
 {
-	tasklet_schedule(&xen_netbk[group].net_rx_tasklet);
+	if (use_kthreads)
+		wake_up(&xen_netbk[group].netbk_action_wq);
+	else
+		tasklet_schedule(&xen_netbk[group].net_rx_tasklet);
 }
 
 static void netbk_tx_pending_timeout(unsigned long group)
 {
-	tasklet_schedule(&xen_netbk[group].net_tx_tasklet);
+	if (use_kthreads)
+		wake_up(&xen_netbk[group].netbk_action_wq);
+	else
+		tasklet_schedule(&xen_netbk[group].net_tx_tasklet);
 }
 
 struct net_device_stats *netif_be_get_stats(struct net_device *dev)
@@ -1506,7 +1530,10 @@ static void net_tx_action(unsigned long 
 			continue;
 		}
 
-		netif_rx(skb);
+		if (use_kthreads)
+			netif_rx_ni(skb);
+		else
+			netif_rx(skb);
 		netif->dev->last_rx = jiffies;
 	}
 
@@ -1532,7 +1559,10 @@ static void netif_idx_release(struct xen
 	netbk->dealloc_prod++;
 	spin_unlock_irqrestore(&netbk->release_lock, flags);
 
-	tasklet_schedule(&netbk->net_tx_tasklet);
+	if (use_kthreads)
+		wake_up(&netbk->netbk_action_wq);
+	else
+		tasklet_schedule(&netbk->net_tx_tasklet);
 }
 
 static void netif_page_release(struct page *page, unsigned int order)
@@ -1670,6 +1700,46 @@ static struct irqaction netif_be_dbg_act
 };
 #endif
 
+static inline int rx_work_todo(struct xen_netbk *netbk)
+{
+	return !skb_queue_empty(&netbk->rx_queue);
+}
+
+static inline int tx_work_todo(struct xen_netbk *netbk)
+{
+	if (netbk->dealloc_cons != netbk->dealloc_prod)
+		return 1;
+
+	if (nr_pending_reqs(netbk) + MAX_SKB_FRAGS < MAX_PENDING_REQS &&
+	    !list_empty(&netbk->net_schedule_list))
+		return 1;
+
+	return 0;
+}
+
+static int netbk_action_thread(void *index)
+{
+	unsigned long group = (unsigned long)index;
+	struct xen_netbk *netbk = &xen_netbk[group];
+
+	while (!kthread_should_stop()) {
+		wait_event_interruptible(netbk->netbk_action_wq,
+					 rx_work_todo(netbk) ||
+					 tx_work_todo(netbk) ||
+					 kthread_should_stop());
+		cond_resched();
+
+		if (rx_work_todo(netbk))
+			net_rx_action(group);
+
+		if (tx_work_todo(netbk))
+			net_tx_action(group);
+	}
+
+	return 0;
+}
+
+
 static int __init netback_init(void)
 {
 	unsigned int i, group;
@@ -1697,8 +1767,26 @@ static int __init netback_init(void)
 	for (group = 0; group < netbk_nr_groups; group++) {
 		struct xen_netbk *netbk = &xen_netbk[group];
 
-		tasklet_init(&netbk->net_tx_tasklet, net_tx_action, group);
-		tasklet_init(&netbk->net_rx_tasklet, net_rx_action, group);
+		if (use_kthreads) {
+			init_waitqueue_head(&netbk->netbk_action_wq);
+			netbk->task = kthread_create(netbk_action_thread,
+						     (void *)(long)group,
+						     "netback/%u", group);
+
+			if (!IS_ERR(netbk->task)) {
+				if (bind_threads)
+					kthread_bind(netbk->task, group);
+				wake_up_process(netbk->task);
+			} else {
+				printk(KERN_ALERT
+				       "kthread_create() fails at netback\n");
+				rc = PTR_ERR(netbk->task);
+				goto failed_init;
+			}
+		} else {
+			tasklet_init(&netbk->net_tx_tasklet, net_tx_action, group);
+			tasklet_init(&netbk->net_rx_tasklet, net_rx_action, group);
+		}
 
 		skb_queue_head_init(&netbk->rx_queue);
 		skb_queue_head_init(&netbk->tx_queue);
@@ -1762,8 +1850,11 @@ failed_init:
 	while (group-- > 0) {
 		struct xen_netbk *netbk = &xen_netbk[group];
 
-		free_empty_pages_and_pagevec(netbk->mmap_pages,
-					     MAX_PENDING_REQS);
+		if (use_kthreads && netbk->task && !IS_ERR(netbk->task))
+			kthread_stop(netbk->task);
+		if (netbk->mmap_pages)
+			free_empty_pages_and_pagevec(netbk->mmap_pages,
+						     MAX_PENDING_REQS);
 		del_timer(&netbk->tx_pending_timer);
 		del_timer(&netbk->net_timer);
 	}
