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

[Ming Lei <ming.lei@redhat.com>]

On Tue, Aug 31, 2021 at 10:27:28AM +0800, luojiaxing wrote:
> Hi, Ming
> 
> 
> Sorry to reply so late, This issue occur in low probability,
> 
> so it take some time to confirm.
> 
> 
> On 2021/8/26 15:29, Ming Lei wrote:
> > On Thu, Aug 26, 2021 at 11:00:34AM +0800, luojiaxing wrote:
> > > 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
> > What is your io scheduler? I guess it is deadline,
> 
> 
> yes
> 
> 
> >   and can you observe
> > such issue by switching to none?
> 
> 
> Yes, it happen when switched to none
> 
> 
> > 
> > The tricky thing is that one request dumped may be re-allocated to other tag
> > after returning from bt_tags_iter().
> > 
> > > 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.
> > Yeah, we know this kind of handling, but not see it as issue.
> > 
> > > 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.
> > I guess the race is triggered when 2808 is just assigned, meantime
> > ->rqs[] isn't updated.
> 
> 
> As we shared tag between hctx, so if 2808 was assinged to other hctx.
> 
> So previous hctx's rqs will not updated。
> 
> 
> > > 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.
> > Can you try the following patch?
> 
> 
> I tested it. it can fix the bug.
> 
> 
> However, if there is still a problem in the following scenario? For example,
> driver tag 0 is assigned
> 
> to rq0 in hctx0, and reclaimed after rq completed. Next time driver tag 0 is
> still assigned to rq0 but
> 
> in hctx1. So at this time,  bt_tags_iter will still got two rqs.

Each hctx has its own rq pool so far, so no such issue you worried.

John's patch works towards sharing rq pool among hctxs in case of
shared sbitmap, not merged yet, but ->rqs[] should be shared too, still
no such issue.

Follows the revised patch for handling the stale request in ->rqs[] issue:


diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 86f87346232a..ff5caeb82542 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -208,7 +208,7 @@ static struct request *blk_mq_find_and_get_req(struct blk_mq_tags *tags,
 
 	spin_lock_irqsave(&tags->lock, flags);
 	rq = tags->rqs[bitnr];
-	if (!rq || !refcount_inc_not_zero(&rq->ref))
+	if (!rq || rq->tag != bitnr || !refcount_inc_not_zero(&rq->ref))
 		rq = NULL;
 	spin_unlock_irqrestore(&tags->lock, flags);
 	return rq;


Thanks,
Ming