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

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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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