[PATCH RFC 03/12] xen-blkfront: switch from llist to list

[Roger Pau Monne]

Replace the use of llist with list.

llist_for_each_entry_safe can trigger a bug in GCC 4.1, so it's best
to remove it and use a doubly linked list, which is used extensively
in the kernel already.

Specifically this bug can be triggered by hot-unplugging a disk,
either by doing xm block-detach or by save/restore cycle.

BUG: unable to handle kernel paging request at fffffffffffffff0
IP: [<ffffffffa0047223>] blkif_free+0x63/0x130 [xen_blkfront]
The crash call trace is:
	...
bad_area_nosemaphore+0x13/0x20
do_page_fault+0x25e/0x4b0
page_fault+0x25/0x30
? blkif_free+0x63/0x130 [xen_blkfront]
blkfront_resume+0x46/0xa0 [xen_blkfront]
xenbus_dev_resume+0x6c/0x140
pm_op+0x192/0x1b0
device_resume+0x82/0x1e0
dpm_resume+0xc9/0x1a0
dpm_resume_end+0x15/0x30
do_suspend+0x117/0x1e0

When drilling down to the assembler code, on newer GCC it does
.L29:
        cmpq    $-16, %r12      #, persistent_gnt check
        je      .L30    	#, out of the loop
.L25:
	... code in the loop
        testq   %r13, %r13      # n
        je      .L29    	#, back to the top of the loop
        cmpq    $-16, %r12      #, persistent_gnt check
        movq    16(%r12), %r13  # <variable>.node.next, n
        jne     .L25    	#,	back to the top of the loop
.L30:

While on GCC 4.1, it is:
L78:
	... code in the loop
	testq   %r13, %r13      # n
        je      .L78    #,	back to the top of the loop
        movq    16(%rbx), %r13  # <variable>.node.next, n
        jmp     .L78    #,	back to the top of the loop

Which basically means that the exit loop condition instead of
being:

	&(pos)->member != NULL;

is:
	;

which makes the loop unbound.

Since we always manipulate the list while holding the io_lock, there's
no need for additional locking (llist used previously is safe to use
concurrently without additional locking).

Should be backported to 3.8 stable.

Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
[Part of the description]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxx
---
 drivers/block/xen-blkfront.c |   41 ++++++++++++++++++-----------------------
 1 files changed, 18 insertions(+), 23 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index c3dae2e..2e39eaf 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -44,7 +44,7 @@
 #include <linux/mutex.h>
 #include <linux/scatterlist.h>
 #include <linux/bitmap.h>
-#include <linux/llist.h>
+#include <linux/list.h>
 
 #include <xen/xen.h>
 #include <xen/xenbus.h>
@@ -68,7 +68,7 @@ enum blkif_state {
 struct grant {
 	grant_ref_t gref;
 	unsigned long pfn;
-	struct llist_node node;
+	struct list_head node;
 };
 
 struct blk_shadow {
@@ -105,7 +105,7 @@ struct blkfront_info
 	struct work_struct work;
 	struct gnttab_free_callback callback;
 	struct blk_shadow shadow[BLK_RING_SIZE];
-	struct llist_head persistent_gnts;
+	struct list_head persistent_gnts;
 	unsigned int persistent_gnts_c;
 	unsigned long shadow_free;
 	unsigned int feature_flush;
@@ -371,10 +371,11 @@ static int blkif_queue_request(struct request *req)
 			lsect = fsect + (sg->length >> 9) - 1;
 
 			if (info->persistent_gnts_c) {
-				BUG_ON(llist_empty(&info->persistent_gnts));
-				gnt_list_entry = llist_entry(
-					llist_del_first(&info->persistent_gnts),
-					struct grant, node);
+				BUG_ON(list_empty(&info->persistent_gnts));
+				gnt_list_entry = list_first_entry(
+				                      &info->persistent_gnts,
+				                      struct grant, node);
+				list_del(&gnt_list_entry->node);
 
 				ref = gnt_list_entry->gref;
 				buffer_mfn = pfn_to_mfn(gnt_list_entry->pfn);
@@ -790,9 +791,8 @@ static void blkif_restart_queue(struct work_struct *work)
 
 static void blkif_free(struct blkfront_info *info, int suspend)
 {
-	struct llist_node *all_gnts;
-	struct grant *persistent_gnt, *tmp;
-	struct llist_node *n;
+	struct grant *persistent_gnt;
+	struct grant *n;
 
 	/* Prevent new requests being issued until we fix things up. */
 	spin_lock_irq(&info->io_lock);
@@ -804,20 +804,15 @@ static void blkif_free(struct blkfront_info *info, int suspend)
 
 	/* Remove all persistent grants */
 	if (info->persistent_gnts_c) {
-		all_gnts = llist_del_all(&info->persistent_gnts);
-		persistent_gnt = llist_entry(all_gnts, typeof(*(persistent_gnt)), node);
-		while (persistent_gnt) {
+		list_for_each_entry_safe(persistent_gnt, n,
+		                         &info->persistent_gnts, node) {
+			list_del(&persistent_gnt->node);
 			gnttab_end_foreign_access(persistent_gnt->gref, 0, 0UL);
 			__free_page(pfn_to_page(persistent_gnt->pfn));
-			tmp = persistent_gnt;
-			n = persistent_gnt->node.next;
-			if (n)
-				persistent_gnt = llist_entry(n, typeof(*(persistent_gnt)), node);
-			else
-				persistent_gnt = NULL;
-			kfree(tmp);
+			kfree(persistent_gnt);
+			info->persistent_gnts_c--;
 		}
-		info->persistent_gnts_c = 0;
+		BUG_ON(info->persistent_gnts_c != 0);
 	}
 
 	/* No more gnttab callback work. */
@@ -875,7 +870,7 @@ static void blkif_completion(struct blk_shadow *s, struct blkfront_info *info,
 	}
 	/* Add the persistent grant into the list of free grants */
 	for (i = 0; i < s->req.u.rw.nr_segments; i++) {
-		llist_add(&s->grants_used[i]->node, &info->persistent_gnts);
+		list_add(&s->grants_used[i]->node, &info->persistent_gnts);
 		info->persistent_gnts_c++;
 	}
 }
@@ -1171,7 +1166,7 @@ static int blkfront_probe(struct xenbus_device *dev,
 	spin_lock_init(&info->io_lock);
 	info->xbdev = dev;
 	info->vdevice = vdevice;
-	init_llist_head(&info->persistent_gnts);
+	INIT_LIST_HEAD(&info->persistent_gnts);
 	info->persistent_gnts_c = 0;
 	info->connected = BLKIF_STATE_DISCONNECTED;
 	INIT_WORK(&info->work, blkif_restart_queue);
-- 
1.7.7.5 (Apple Git-26)

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