Re: REGRESSION: 37f4a24c2469: blk-mq: centralise related handling into blk_mq_get_driver_tag

Ming Lei
2020-09-15T23:10:24Z

On Tue, Sep 15, 2020 at 06:45:41PM -0400, Theodore Y. Ts'o wrote:
> On Tue, Sep 15, 2020 at 03:33:03PM +0800, Ming Lei wrote:
> > Hi Theodore,
> >
> > On Tue, Sep 15, 2020 at 12:45:19AM -0400, Theodore Y. Ts'o wrote:
> > > On Thu, Sep 03, 2020 at 11:55:28PM -0400, Theodore Y. Ts'o wrote:
> > > > Worse, right now, -rc1 and -rc2 is causing random crashes in my
> > > > gce-xfstests framework. Sometimes it happens before we've run even a
> > > > single xfstests; sometimes it happens after we have successfully
> > > > completed all of the tests, and we're doing a shutdown of the VM under
> > > > test. Other times it happens in the middle of a test run. Given that
> > > > I'm seeing this at -rc1, which is before my late ext4 pull request to
> > > > Linus, it's probably not an ext4 related bug. But it also means that
> > > > I'm partially blind in terms of my kernel testing at the moment. So I
> > > > can't even tell Linus that I've run lots of tests and I'm 100%
> > > > confident your one-line change is 100% safe.
> > >
> > > I was finally able to bisect it down to the commit:
> > >
> > > 37f4a24c2469: blk-mq: centralise related handling into blk_mq_get_driver_tag
> >
> > 37f4a24c2469 has been reverted in:
> >
> > 4e2f62e566b5 Revert "blk-mq: put driver tag when this request is completed"
> >
> > And later the patch is committed as the following after being fixed:
> >
> > 568f27006577 blk-mq: centralise related handling into blk_mq_get_driver_tag
> >
> > So can you reproduce the issue by running kernel of commit 568f27006577?
>
> Yes. And things work fine if I try 4e2f62e566b5.
>
> > If yes, can the issue be fixed by reverting 568f27006577?
>
> The problem is it's a bit tricky to revert 568f27006577, since there
> is a merge conflict in blk_kick_flush(). I attempted to do the bisect
> manually here, but it's clearly not right since the kernel is not
> booting after the revert:
>
> https://github.com/tytso/ext4/commit/1e67516382a33da2c9d483b860ac4ec2ad390870
>
> branch:
>
> https://github.com/tytso/ext4/tree/manual-revert-of-568f27006577
>
> Can you send me a patch which correctly reverts 568f27006577? I can
> try it against -rc1 .. -rc4, whichever is most convenient.

Please test the following revert patch against -rc4.

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 53abb5c73d99..24c208d21793 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -219,6 +219,7 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
struct request *rq, *n;
unsigned long flags = 0;
struct blk_flush_queue *fq = blk_get_flush_queue(q, flush_rq->mq_ctx);
+ struct blk_mq_hw_ctx *hctx;

blk_account_io_flush(flush_rq);

@@ -234,11 +235,13 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
if (fq->rq_status != BLK_STS_OK)
error = fq->rq_status;

+ hctx = flush_rq->mq_hctx;
if (!q->elevator) {
- flush_rq->tag = BLK_MQ_NO_TAG;
+ blk_mq_tag_set_rq(hctx, flush_rq->tag, fq->orig_rq);
+ flush_rq->tag = -1;
} else {
blk_mq_put_driver_tag(flush_rq);
- flush_rq->internal_tag = BLK_MQ_NO_TAG;
+ flush_rq->internal_tag = -1;
}

running = &fq->flush_queue[fq->flush_running_idx];
@@ -309,16 +312,12 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
flush_rq->mq_hctx = first_rq->mq_hctx;

if (!q->elevator) {
+ fq->orig_rq = first_rq;
flush_rq->tag = first_rq->tag;
-
- /*
- * We borrow data request's driver tag, so have to mark
- * this flush request as INFLIGHT for avoiding double
- * account of this driver tag
- */
- flush_rq->rq_flags |= RQF_MQ_INFLIGHT;
- } else
+ blk_mq_tag_set_rq(flush_rq->mq_hctx, first_rq->tag, flush_rq);
+ } else {
flush_rq->internal_tag = first_rq->internal_tag;
+ }

flush_rq->cmd_flags = REQ_OP_FLUSH | REQ_PREFLUSH;
flush_rq->cmd_flags |= (flags & REQ_DRV) | (flags & REQ_FAILFAST_MASK);
diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index b1acac518c4e..3945c7f5b944 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h
@@ -101,6 +101,18 @@ static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
return atomic_read(&hctx->nr_active) < depth;
}

+/*
+ * This helper should only be used for flush request to share tag
+ * with the request cloned from, and both the two requests can't be
+ * in flight at the same time. The caller has to make sure the tag
+ * can't be freed.
+ */
+static inline void blk_mq_tag_set_rq(struct blk_mq_hw_ctx *hctx,
+ unsigned int tag, struct request *rq)
+{
+ hctx->tags->rqs[tag] = rq;
+}
+
static inline bool blk_mq_tag_is_reserved(struct blk_mq_tags *tags,
unsigned int tag)
{
diff --git a/block/blk-mq.c b/block/blk-mq.c
index b3d2785eefe9..feb3d5c0a1c6 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -277,20 +277,26 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
{
struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
struct request *rq = tags->static_rqs[tag];
+ req_flags_t rq_flags = 0;

if (data->q->elevator) {
rq->tag = BLK_MQ_NO_TAG;
rq->internal_tag = tag;
} else {
+ if (data->hctx->flags & BLK_MQ_F_TAG_SHARED) {
+ rq_flags = RQF_MQ_INFLIGHT;
+ atomic_inc(&data->hctx->nr_active);
+ }
rq->tag = tag;
rq->internal_tag = BLK_MQ_NO_TAG;
+ data->hctx->tags->rqs[rq->tag] = rq;
}

/* csd/requeue_work/fifo_time is initialized before use */
rq->q = data->q;
rq->mq_ctx = data->ctx;
rq->mq_hctx = data->hctx;
- rq->rq_flags = 0;
+ rq->rq_flags = rq_flags;
rq->cmd_flags = data->cmd_flags;
if (data->flags & BLK_MQ_REQ_PREEMPT)
rq->rq_flags |= RQF_PREEMPT;
@@ -1098,10 +1104,9 @@ static bool __blk_mq_get_driver_tag(struct request *rq)
{
struct sbitmap_queue *bt = &rq->mq_hctx->tags->bitmap_tags;
unsigned int tag_offset = rq->mq_hctx->tags->nr_reserved_tags;
+ bool shared = blk_mq_tag_busy(rq->mq_hctx);
int tag;

- blk_mq_tag_busy(rq->mq_hctx);
-
if (blk_mq_tag_is_reserved(rq->mq_hctx->sched_tags, rq->internal_tag)) {
bt = &rq->mq_hctx->tags->breserved_tags;
tag_offset = 0;
@@ -1114,23 +1119,19 @@ static bool __blk_mq_get_driver_tag(struct request *rq)
return false;

rq->tag = tag + tag_offset;
+ if (shared) {
+ rq->rq_flags |= RQF_MQ_INFLIGHT;
+ atomic_inc(&rq->mq_hctx->nr_active);
+ }
+ rq->mq_hctx->tags->rqs[rq->tag] = rq;
return true;
}

static bool blk_mq_get_driver_tag(struct request *rq)
{
- struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
-
- if (rq->tag == BLK_MQ_NO_TAG && !__blk_mq_get_driver_tag(rq))
- return false;
-
- if ((hctx->flags & BLK_MQ_F_TAG_SHARED) &&
- !(rq->rq_flags & RQF_MQ_INFLIGHT)) {
- rq->rq_flags |= RQF_MQ_INFLIGHT;
- atomic_inc(&hctx->nr_active);
- }
- hctx->tags->rqs[rq->tag] = rq;
- return true;
+ if (rq->tag != BLK_MQ_NO_TAG)
+ return true;
+ return __blk_mq_get_driver_tag(rq);
}

static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode,
diff --git a/block/blk.h b/block/blk.h
index 49e2928a1632..5ed35a02bc7f 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -25,6 +25,11 @@ struct blk_flush_queue {
struct list_head flush_data_in_flight;
struct request *flush_rq;

+ /*
+ * flush_rq shares tag with this rq, both can't be active
+ * at the same time
+ */
+ struct request *orig_rq;
struct lock_class_key key;
spinlock_t mq_flush_lock;
};

thanks,
Ming