rq pointer in tags->rqs[] is not cleared in time and make SCSI error handle can not be triggered

[luojiaxing <luojiaxing@huawei.com>]

Dear all:


I meet some problem when test hisi_sas driver(under SCSI) based on 
5.14-rc4 kernel, it's found that error handle can not be triggered after

abnormal IO occur in some test with a low probability. For example, 
circularly run disk hardreset or disable all local phy of expander when 
running fio.


We add some tracepoint and print to see what happen, and we got the 
following information:

(1).print rq and rq_state at bt_tags_iter() to confirm how many IOs is 
running now.

<4>[  897.431182] bt_tags_iter: rqs[2808]: 0xffff202007bd3000; rq_state: 1
<4>[  897.437514] bt_tags_iter: rqs[3185]: 0xffff0020c5261e00; rq_state: 1
<4>[  897.443841] bt_tags_iter: rqs[3612]: 0xffff00212f242a00; rq_state: 1
<4>[  897.450167] bt_tags_iter: rqs[2808]: 0xffff00211d208100; rq_state: 1
<4>[  897.456492] bt_tags_iter: rqs[2921]: 0xffff00211d208100; rq_state: 1
<4>[  897.462818] bt_tags_iter: rqs[1214]: 0xffff002151d21b00; rq_state: 1
<4>[  897.469143] bt_tags_iter: rqs[2648]: 0xffff0020c4bfa200; rq_state: 1

The preceding information show that rq with tag[2808] is found in 
different hctx by bt_tags_iter() and with different pointer saved in 
tags->rqs[].

And tag[2808] own the same pointer value saved in rqs[] with tag[2921]. 
It's wrong because our driver share tag between all hctx, so it's not 
possible

to allocate one tag to different rq.


(2).check tracepoints(temporarily add) in blk_mq_get_driver_tag() and 
blk_mq_put_tag() to see where this tag is come from.

     Line 1322969:            <...>-20189   [013] .... 893.427707: 
blk_mq_get_driver_tag: rqs[2808]: 0xffff00211d208100
     Line 1322997:  irq/1161-hisi_s-7602    [012] d..1 893.427814: 
blk_mq_put_tag_in_free_request: rqs[2808]: 0xffff00211d208100
     Line 1331257:            <...>-20189   [013] .... 893.462663: 
blk_mq_get_driver_tag: rqs[2860]: 0xffff00211d208100
     Line 1331289:  irq/1161-hisi_s-7602    [012] d..1 893.462785: 
blk_mq_put_tag_in_free_request: rqs[2860]: 0xffff00211d208100
     Line 1338493:            <...>-20189   [013] .... 893.493519: 
blk_mq_get_driver_tag: rqs[2921]: 0xffff00211d208100

As we can see this rq is allocated to tag[2808] once, and finially come 
to tag[2921], but rqs[2808] still save the pointer.

There will be no problem until we encounter a rare situation.

For example, tag[2808] is reassigned to another hctx for execution, then 
some IO meet some error.

Before waking up the error handle thread, SCSI compares the values of 
scsi_host_busy() and shost->host_failed.

If the values are different, SCSI waits for the completion of some I/Os. 
According to the print provided by (1), the return value of 
scsi_host_busy() should be 7 for tag [2808] is calculated twice,

and the value of shost->host_failed is 6. As a result, this two values 
are never equal, and error handle cannot be triggered.


A temporary workaround is provided and can solve the problem as:

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 2a37731..e3dc773 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -190,6 +190,7 @@ void blk_mq_put_tag(struct blk_mq_tags *tags, struct 
blk_mq_ctx *ctx,
                 BUG_ON(tag >= tags->nr_reserved_tags);
                 sbitmap_queue_clear(tags->breserved_tags, tag, ctx->cpu);
         }
+       tags->rqs[tag] = NULL;
  }


Since we did not encounter this problem in some previous kernel 
versions, we wondered if the community already knew about the problem or 
could provide some solutions.


Thanks

Jiaxing