Re: [PATCH 3/5] xen-blkfront: switch from llist to list

Roger Pau Monné
2013-03-19T12:57:25Z

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On 19/03/13 13:51, Konrad Rzeszutek Wilk wrote:
> On Mon, Mar 18, 2013 at 05:49:34PM +0100, Roger Pau Monne wrote:
>> 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 $-16, %r12 #, persistent_gnt check
>> movq 16(%r12), %r13 # <variable>.node.next, n
>> jne .L25 #, back to the top of the loop
>> .L30:
>>
>> While on GCC 4.1, it is:
>> L78:
>> ... code in the loop
>> testq %r13, %r13 # n
>> je .L78 #, back to the top of the loop
>> movq 16(%rbx), %r13 # <variable>.node.next, n
>> jmp .L78 #, back to the top of the loop
>>
>> Which basically means that the exit loop condition instead of
>> being:
>>
>> &(pos)->member != NULL;
>>
>> is:
>> ;
>>
>> which makes the loop unbound.
>>
>> Since we always manipulate the list while holding the io_lock, there's
>> no need for additional locking (llist used previously is safe to use
>> concurrently without additional locking).
>>
>> Should be backported to 3.8 stable.
>
> I get
> konrad@phenom:~/linux$ patch -p1 < /tmp/kk
> patching file drivers/block/xen-blkfront.c
> Hunk #1 FAILED at 44.
> Hunk #2 FAILED at 68.
> Hunk #3 FAILED at 105.
> Hunk #4 FAILED at 371.
> patch: **** malformed patch at line 172: ork)
>
>
> and idea why? Could you resent the patch as an attachment please?

Strange, I've used git send-email. I've attached it, but you can also
get them from:

http://xenbits.xen.org/gitweb/?p=people/royger/linux.git;a=shortlog;h=refs/heads/blk-for-3.9

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>From 59a1e9b3ec1cbae76a8ee2ad142255d5d1fc1ae1 Mon Sep 17 00:00:00 2001
From: Roger Pau Monne <roger.pau@citrix.com>
Date: Mon, 25 Feb 2013 10:49:10 +0100
Subject: [PATCH 3/5] xen-blkfront: switch from llist to list
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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 Monné <roger.pau@citrix.com>
[Part of the description]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: xen-devel@lists.xen.org
---
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 9620644..97324cd1 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)