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

Konrad Rzeszutek Wilk
2013-03-19T12:52:00Z

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?

>
> 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)
>