Hello,
On Fri, Sep 04, 2015 at 11:46:02AM +0100, Richard W.M. Jones wrote:
> $ addr2line -e /usr/lib/debug/lib/modules/4.2.0-0.rc3.git4.1.fc24.x86_64/vmlinux ffffffff814107a0
> /usr/src/debug/kernel-4.1.fc24/linux-4.2.0-0.rc3.git4.1.fc24.x86_64/block/blk-throttle.c:1642
>
> 1636 /*
> 1637 * Drain each tg while doing post-order walk on the blkg tree, s 1637 o
> 1638 * that all bios are propagated to td->service_queue. It'd be
> 1639 * better to walk service_queue tree directly but blkg walk is
> 1640 * easier.
> 1641 */
> 1642 blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg)
> 1643 tg_drain_bios(&blkg_to_tg(blkg)->service_queue);
> 1644
>
> Rich.
>
> [ 6.784689] BUG: unable to handle kernel NULL pointer dereference at 0000000000000bb8
> [ 6.787605] IP: [<ffffffff814107a0>] blk_throtl_drain+0x80/0x220
The only struct which is large enough for 0xbb8 offset is
request_queue. Hmm.... can you please try the brute force debug patch
below and report the kernel log after the crash?
Thanks.
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index b231935..09426e4 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1639,8 +1639,22 @@ void blk_throtl_drain(struct request_queue *q)
* better to walk service_queue tree directly but blkg walk is
* easier.
*/
- blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg)
- tg_drain_bios(&blkg_to_tg(blkg)->service_queue);
+ printk("XXX blk_throtl_drain: td=%p ->queue=%p ->root_blkg=%p ->q/blkcg=%p/%p\n",
+ td, td ? td->queue : NULL,
+ (td && td->queue) ? td->queue->root_blkg : NULL,
+ (td && td->queue && td->queue->root_blkg) ? td->queue->root_blkg->q : NULL,
+ (td && td->queue && td->queue->root_blkg) ? td->queue->root_blkg->blkcg : NULL);
+
+ css_for_each_descendant_pre(pos_css, &td->queue->root_blkg->blkcg->css) {
+ printk("XXX pos_css=%p ", pos_css);
+ pr_cont_cgroup_path(pos_css->cgroup);
+ if ((blkg = __blkg_lookup(css_to_blkcg(pos_css),
+ td->queue->root_blkg->q, false))) {
+ pr_cont(" blkg=%p", blkg);
+ tg_drain_bios(&blkg_to_tg(blkg)->service_queue);
+ }
+ pr_cont("\n");
+ }
/* finally, transfer bios from top-level tg's into the td */
tg_drain_bios(&td->service_queue);
--
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