Re: [RFC PATCH v2 2/2] blk-mq: Lockout tagset iter when freeing rqs

From: Bart Van Assche
Date: Tue Dec 22 2020 - 11:17:53 EST


On 12/22/20 3:15 AM, John Garry wrote:
So then we could have something like this:

---8<---

 -435,9 +444,13 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
    if (!blk_mq_hw_queue_mapped(hctx))
            continue;

+    while (!atomic_inc_not_zero(&tags->iter_usage_counter));
+
    if (tags->nr_reserved_tags)
        bt_for_each(hctx, tags->breserved_tags, fn, priv, true);
    bt_for_each(hctx, tags->bitmap_tags, fn, priv, false);

+    atomic_dec(&tags->iter_usage_counter);
}

blk_queue_exit(q);

--->8---

And similar for blk_mq_tagset_busy_iter(). How about it?

Are there any blk_mq_tagset_busy_iter() calls that happen from a context where the tag set can disappear while that function is in progress?

Some blk_mq_tagset_busy_iter() calls happen from a context where it is not allowed to sleep but also where it is guaranteed that the tag set won't disappear, e.g. the call from inside sdk_mq_queue_rq().

How about using a mutex inside blk_mq_queue_tag_busy_iter() instead? As far as I can see all blk_mq_queue_tag_busy_iter() happen from a context where it is allowed to sleep.

Thanks,

Bart.