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[Xen-devel] [PATCH] xen-blkfront: switch from llist to list

From: Roger Pau Monne <roger.pau@xxxxxxxxxx>

Date: Fri, 15 Feb 2013 19:03:01 +0100

Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxx, Roger Pau Monne <roger.pau@xxxxxxxxxx>

Delivery-date: Fri, 15 Feb 2013 18:05:14 +0000

List-id: Xen developer discussion <xen-devel.lists.xen.org>

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 MonnÃ <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 | 27 ++++++++++++++------------- include/linux/llist.h | 25 ------------------------- 2 files changed, 14 insertions(+), 38 deletions(-) diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c index 11043c1..01b91a5 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; - struct llist_node *n; + struct grant *n; /* Prevent new requests being issued until we fix things up. */ spin_lock_irq(&info->io_lock); @@ -804,8 +804,9 @@ 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) { + 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)); kfree(persistent_gnt); @@ -868,7 +869,7 @@ 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++; } } @@ -1164,7 +1165,7 @@ 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); 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 * -- 1.7.7.5 (Apple Git-26) _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel

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