On 07/02/2017 04:45 AM, Max Gurtovoy wrote:

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 ?

Right, this is clearly a different issue and my first thought as well
was that it's a missing quiesce of the queue. We're iterating the tags
when they are being torn down.

Looks like Sagi's patch fixes the issue, so I'm considering this one
resolved.

-- 
Jens Axboe