On 02/07/17 13:45, Max Gurtovoy wrote:

quoted

On 6/30/2017 8:26 PM, Jens Axboe wrote:

quoted

Hi Max,

Hi Jens,

quoted

I remembered you reporting this. I think this is a regression introduced
with the scheduling, since ->rqs[] isn't static anymore. ->static_rqs[]
is, but that's not indexable by the tag we find. So I think we need to
guard those with a NULL check. The actual requests themselves are
static, so we know the memory itself isn't going away. But if we race
with completion, we could find a NULL there, validly.

Since you could reproduce it, can you try the below?

I still can repro the null deref with this patch applied.

quoted

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index d0be72ccb091..b856b2827157 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c

@@ -214,7 +214,7 @@ static bool bt_iter(struct sbitmap *bitmap,

unsigned int bitnr, void *data)
         bitnr += tags->nr_reserved_tags;
     rq = tags->rqs[bitnr];

-    if (rq->q == hctx->queue)
+    if (rq && rq->q == hctx->queue)
         iter_data->fn(hctx, rq, iter_data->data, reserved);
     return true;
 }

@@ -249,8 +249,8 @@ static bool bt_tags_iter(struct sbitmap *bitmap,

unsigned int bitnr, void *data)
     if (!reserved)
         bitnr += tags->nr_reserved_tags;
     rq = tags->rqs[bitnr];
-
-    iter_data->fn(rq, iter_data->data, reserved);
+    if (rq)
+        iter_data->fn(rq, iter_data->data, reserved);
     return true;
 }

see the attached file for dmesg output.

output of gdb:

(gdb) list *(blk_mq_flush_busy_ctxs+0x48)
0xffffffff8127b108 is in blk_mq_flush_busy_ctxs
(./include/linux/sbitmap.h:234).
229
230             for (i = 0; i < sb->map_nr; i++) {
231                     struct sbitmap_word *word = &sb->map[i];
232                     unsigned int off, nr;
233
234                     if (!word->word)
235                             continue;
236
237                     nr = 0;
238                     off = i << sb->shift;


when I change the "if (!word->word)" to  "if (word && !word->word)"
I can get null deref at "nr = find_next_bit(&word->word, word->depth,
nr);". Seems like somehow word becomes NULL.

Adding the linux-nvme guys too.
Sagi has mentioned that this can be null only if we remove the tagset
while I/O is trying to get a tag and when killing the target we get into
error recovery and periodic reconnects, which does _NOT_ include freeing
the tagset, so this is probably the admin tagset.

Sagi,
you've mention a patch for centrelizing the treatment of the admin
tagset to the nvme core. I think I missed this patch, so can you please
send a pointer to it and I'll check if it helps ?

Hmm,

In the above flow we should not be freeing the tag_set, not on admin as
well. The target keep removing namespaces and finally removes the
subsystem which generates a error recovery flow. What we at least try
to do is:

1. mark rdma queues as not live
2. stop all the sw queues (admin and io)
3. fail inflight I/Os
4. restart all sw queues (to fast fail until we recover)

We shouldn't be freeing the tagsets (although we might update them
when we recover and cpu map changed - which I don't think is happening).

However, I do see a difference between bt_tags_for_each
and blk_mq_flush_busy_ctxs (checks tags->rqs not being NULL).

Unrelated to this I think we should quiesce/unquiesce the admin_q
instead of stop/start because it respects the submission path rcu [1].

It might hide the issue, but given that we never free the tagset its
seems like it's not in nvme-rdma (max, can you see if this makes the
issue go away?)

[1]:
--

diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
index e3996db22738..094873a4ee38 100644
--- a/drivers/nvme/host/rdma.c
+++ b/drivers/nvme/host/rdma.c

@@ -785,7 +785,7 @@ static void nvme_rdma_error_recovery_work(struct

work_struct *work)

        if (ctrl->ctrl.queue_count > 1)
                nvme_stop_queues(&ctrl->ctrl);
-       blk_mq_stop_hw_queues(ctrl->ctrl.admin_q);
+       blk_mq_quiesce_queue(ctrl->ctrl.admin_q);

        /* We must take care of fastfail/requeue all our inflight requests
*/
        if (ctrl->ctrl.queue_count > 1)

@@ -798,7 +798,8 @@ static void nvme_rdma_error_recovery_work(struct

work_struct *work)
         * queues are not a live anymore, so restart the queues to fail fast
         * new IO
         */
-       blk_mq_start_stopped_hw_queues(ctrl->ctrl.admin_q, true);
+       blk_mq_unquiesce_queue(ctrl->ctrl.admin_q);
+       blk_mq_kick_requeue_list(ctrl->ctrl.admin_q);
        nvme_start_queues(&ctrl->ctrl);

        nvme_rdma_reconnect_or_remove(ctrl);

@@ -1651,7 +1652,7 @@ static void nvme_rdma_shutdown_ctrl(struct

nvme_rdma_ctrl *ctrl)
        if (test_bit(NVME_RDMA_Q_LIVE, &ctrl->queues[0].flags))
                nvme_shutdown_ctrl(&ctrl->ctrl);

-       blk_mq_stop_hw_queues(ctrl->ctrl.admin_q);
+       blk_mq_quiesce_queue(ctrl->ctrl.admin_q);
        blk_mq_tagset_busy_iter(&ctrl->admin_tag_set,
                                nvme_cancel_request, &ctrl->ctrl);
        nvme_rdma_destroy_admin_queue(ctrl);