On Tue, Aug 15, 2023 at 02:04:42PM +0800, chengming.zhou@xxxxxxxxx wrote:
> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>
> When doing io_uring benchmark on /dev/nullb0, it's easy to crash the
> kernel if poll requests timeout triggered, as reported by David. [1]
Just be curious, how is the timeout triggered when running
"./fio/t/io_uring -r20 /dev/nullb0"?
David mentioned that the issue is triggered in 6.5-rc1, maybe one
regression?
>
> BUG: kernel NULL pointer dereference, address: 0000000000000008
> Workqueue: kblockd blk_mq_timeout_work
> RIP: 0010:null_timeout_rq+0x4e/0x91
> Call Trace:
> ? __die_body+0x1a/0x5c
> ? page_fault_oops+0x6f/0x9c
> ? kernelmode_fixup_or_oops+0xc6/0xd6
> ? __bad_area_nosemaphore+0x44/0x1eb
> ? exc_page_fault+0xe2/0xf4
> ? asm_exc_page_fault+0x22/0x30
> ? null_timeout_rq+0x4e/0x91
> blk_mq_handle_expired+0x31/0x4b
> bt_iter+0x68/0x84
> ? bt_tags_iter+0x81/0x81
> __sbitmap_for_each_set.constprop.0+0xb0/0xf2
> ? __blk_mq_complete_request_remote+0xf/0xf
> bt_for_each+0x46/0x64
> ? __blk_mq_complete_request_remote+0xf/0xf
> ? percpu_ref_get_many+0xc/0x2a
> blk_mq_queue_tag_busy_iter+0x14d/0x18e
> blk_mq_timeout_work+0x95/0x127
> process_one_work+0x185/0x263
> worker_thread+0x1b5/0x227
> ? rescuer_thread+0x287/0x287
> kthread+0xfa/0x102
> ? kthread_complete_and_exit+0x1b/0x1b
> ret_from_fork+0x22/0x30
>
> This is indeed a race problem between null_timeout_rq() and null_poll().
>
> null_poll() null_timeout_rq()
> spin_lock(&nq->poll_lock)
> list_splice_init(&nq->poll_list, &list)
> spin_unlock(&nq->poll_lock)
>
> while (!list_empty(&list))
> req = list_first_entry()
> list_del_init()
> ...
> blk_mq_add_to_batch()
> // req->rq_next = NULL
> spin_lock(&nq->poll_lock)
>
> // rq->queuelist->next == NULL
> list_del_init(&rq->queuelist)
>
> spin_unlock(&nq->poll_lock)
>
> What's worse is that we don't call blk_mq_complete_request_remote()
> before blk_mq_add_to_batch(), so these completed requests have wrong
> rq->state == MQ_RQ_IN_FLIGHT. We can easily check this using bpftrace:
>
> ```
> bpftrace -e 'kretfunc:null_blk:null_poll {
> $iob=(struct io_comp_batch *)args->iob;
> @[$iob->req_list->state]=count();
> }'
>
> @[1]: 51708
> ```
>
> Fix these problems by setting requests state to MQ_RQ_COMPLETE under
> nq->poll_lock protection, in which null_timeout_rq() can safely detect
> this race and early return.
>
> [1] https://lore.kernel.org/all/3893581.1691785261@xxxxxxxxxxxxxxxxxxxxxx/
>
> Fixes: 0a593fbbc245 ("null_blk: poll queue support")
> Reported-by: David Howells <dhowells@xxxxxxxxxx>
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> ---
> drivers/block/null_blk/main.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
> index 864013019d6b..968090935eb2 100644
> --- a/drivers/block/null_blk/main.c
> +++ b/drivers/block/null_blk/main.c
> @@ -1643,9 +1643,12 @@ static int null_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
> struct nullb_queue *nq = hctx->driver_data;
> LIST_HEAD(list);
> int nr = 0;
> + struct request *rq;
>
> spin_lock(&nq->poll_lock);
> list_splice_init(&nq->poll_list, &list);
> + list_for_each_entry(rq, &list, queuelist)
> + blk_mq_set_request_complete(rq);
> spin_unlock(&nq->poll_lock);
>
> while (!list_empty(&list)) {
> @@ -1671,16 +1674,21 @@ static enum blk_eh_timer_return null_timeout_rq(struct request *rq)
> struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> struct nullb_cmd *cmd = blk_mq_rq_to_pdu(rq);
>
> - pr_info("rq %p timed out\n", rq);
> -
> if (hctx->type == HCTX_TYPE_POLL) {
> struct nullb_queue *nq = hctx->driver_data;
>
> spin_lock(&nq->poll_lock);
> + /* The request may have completed meanwhile. */
> + if (blk_mq_request_completed(rq)) {
> + spin_unlock(&nq->poll_lock);
> + return BLK_EH_DONE;
> + }
> list_del_init(&rq->queuelist);
> spin_unlock(&nq->poll_lock);
> }
I think null_process_cmd() is needed for un-completed request.
Thanks,
Ming
