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Message-ID: <20130213173312.GA20866@phenom.dumpdata.com>
Date: Wed, 13 Feb 2013 12:33:12 -0500
From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To: Roger Pau Monne <roger.pau@...rix.com>
Cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xen.org
Subject: Re: [PATCH] xen-blkfront: drop the use of llist_for_each_entry_safe
On Mon, Feb 11, 2013 at 05:10:58PM +0100, Roger Pau Monne wrote:
> Replace llist_for_each_entry_safe with a while loop and
> llist_del_first.
>
> llist_for_each_entry_safe can trigger a bug in GCC 4.1, so it's best
> to remove it and use a while loop and llist_del_first (which is
> already in llist.h).
I spruced this up a bit:
commit f73be4e09510cda1457ab96abf356e664ba96d4a
Author: Roger Pau Monne <roger.pau@...rix.com>
Date: Mon Feb 11 17:10:58 2013 +0100
xen-blkfront: drop the use of llist_for_each_entry_safe
Replace llist_for_each_entry_safe with a while loop and
llist_del_first.
llist_for_each_entry_safe can trigger a bug in GCC 4.1, so it's best
to remove it and use a while loop and llist_del_first (which is
already in llist.h).
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 xen-blkfront is the only user of the llist_for_each_entry_safe
macro remove it from llist.h.
Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc: xen-devel@...ts.xen.org
[v1: Spruced the description]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 11043c1..c325062 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -790,7 +790,6 @@ 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;
struct llist_node *n;
@@ -804,8 +803,8 @@ 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);
- llist_for_each_entry_safe(persistent_gnt, n, all_gnts, node) {
+ while ((n = llist_del_first(&info->persistent_gnts)) != NULL) {
+ persistent_gnt = llist_entry(n, struct grant, node);
gnttab_end_foreign_access(persistent_gnt->gref, 0, 0UL);
__free_page(pfn_to_page(persistent_gnt->pfn));
kfree(persistent_gnt);
diff --git a/include/linux/llist.h b/include/linux/llist.h
index d0ab98f..a5199f6 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -125,31 +125,6 @@ static inline void init_llist_head(struct llist_head *list)
(pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))
/**
- * llist_for_each_entry_safe - iterate safely against remove over some entries
- * of lock-less list of given type.
- * @pos: the type * to use as a loop cursor.
- * @n: another type * to use as a temporary storage.
- * @node: the fist entry of deleted list entries.
- * @member: the name of the llist_node with the struct.
- *
- * In general, some entries of the lock-less list can be traversed
- * safely only after being removed from list, so start with an entry
- * instead of list head. This variant allows removal of entries
- * as we iterate.
- *
- * If being used on entries deleted from lock-less list directly, the
- * traverse order is from the newest to the oldest added entry. If
- * you want to traverse from the oldest to the newest, you must
- * reverse the order by yourself before traversing.
- */
-#define llist_for_each_entry_safe(pos, n, node, member) \
- for ((pos) = llist_entry((node), typeof(*(pos)), member), \
- (n) = (pos)->member.next; \
- &(pos)->member != NULL; \
- (pos) = llist_entry(n, typeof(*(pos)), member), \
- (n) = (&(pos)->member != NULL) ? (pos)->member.next : NULL)
-
-/**
* llist_empty - tests whether a lock-less list is empty
* @head: the list to test
*
--
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