Re: [PATCH 1/2] blk-mq: don't complete un-started request in timeout handler

Bart Van Assche
2017-03-15T21:35:31Z

On Thu, 2017-03-16 at 00:22 +0800, Ming Lei wrote:
> On Wed, Mar 15, 2017 at 03:36:31PM +0000, Bart Van Assche wrote:
> > Please have another look at __blk_mq_requeue_request(). In that function
> > the following code occurs: if (test_and_clear_bit(REQ_ATOM_STARTED,
> > &rq->atomic_flags)) { ... }
> >
> > I think the REQ_ATOM_STARTED check in blk_mq_check_expired() races with the
> > test_and_clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags) call in
> > __blk_mq_requeue_request().
>
> OK, this race should only exist in case that the requeue happens after dispatch
> busy, because COMPLETE flag isn't set. And if the requeue is from io completion,
> no such race because COMPLETE flag is set.
>
> One solution I thought of is to call blk_mark_rq_complete() before requeuing
> when dispatch busy happened, but that looks a bit silly. Another way is to
> set STARTED flag just after .queue_rq returns BLK_MQ_RQ_QUEUE_OK, which looks
> reasonable too. Any comments on the 2nd solution?

Sorry but I don't think that would be sufficient. There are several other
scenarios that have not been mentioned above, e.g. that a tag gets freed and
reused after the blk_mq_check_expired() call started and before that function
has had the chance to examine any members of struct request. What is needed in
blk_mq_check_expired() is the following as a single atomic operation:
* Check whether REQ_ATOM_STARTED has been set.
* Check whether REQ_ATOM_COMPLETE has not yet been set.
* If both conditions have been met, set REQ_ATOM_COMPLETE.

I don't think there is another solution than using a single state variable to
represent the REQ_ATOM_STARTED and REQ_ATOM_COMPLETE states instead of two
independent bits. How about the patch below?

Thanks,

Bart.

[PATCH] blk-mq: Fix request state manipulation races

Use a single state variable instead of two separate bits
REQ_ATOM_STARTED and REQ_ATOM_COMPLETE. For blk-mq, make
__blk_mq_finish_request() perform the transition from "complete" to
"not complete" instead of blk_mq_start_request(). Make sure that
blk_mq_check_expired() uses a single atomic operation to test the
"started" and "complete" states and to set the "complete" state.
---
block/blk-core.c           |  6 +++--
block/blk-mq.c             | 67 +++++++++++++++++-----------------------------
block/blk-timeout.c        |  2 +-
block/blk.h                | 21 ++++++++++-----
drivers/scsi/virtio_scsi.c |  2 +-
include/linux/blkdev.h     |  1 +
6 files changed, 47 insertions(+), 52 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 0eeb99ef654f..dff857d2b86b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1305,7 +1305,7 @@ EXPORT_SYMBOL(blk_get_request);
void blk_requeue_request(struct request_queue *q, struct request *rq)
{
blk_delete_timer(rq);
- blk_clear_rq_complete(rq);
+ atomic_set(&rq->state, REQ_NOT_STARTED);
trace_block_rq_requeue(q, rq);
wbt_requeue(q->rq_wb, &rq->issue_stat);

@@ -2477,7 +2477,9 @@ void blk_start_request(struct request *req)
wbt_issue(req->q->rq_wb, &req->issue_stat);
}

- BUG_ON(test_bit(REQ_ATOM_COMPLETE, &req->atomic_flags));
+ WARN_ONCE(atomic_read(&req->state) != REQ_NOT_STARTED,
+   "unexpected request state %d != %d\n",
+   atomic_read(&req->state), REQ_NOT_STARTED);
blk_add_timer(req);
}
EXPORT_SYMBOL(blk_start_request);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 159187a28d66..fe73d5a1ffc3 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -343,7 +343,7 @@ void __blk_mq_finish_request(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
wbt_done(q->rq_wb, &rq->issue_stat);
rq->rq_flags = 0;

- clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
+ atomic_set(&rq->state, REQ_NOT_STARTED);
clear_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags);
if (rq->tag != -1)
blk_mq_put_tag(hctx, hctx->tags, ctx, rq->tag);
@@ -479,7 +479,7 @@ EXPORT_SYMBOL(blk_mq_complete_request);

int blk_mq_request_started(struct request *rq)
{
- return test_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
+ return atomic_read(&rq->state) == REQ_STARTED;
}
EXPORT_SYMBOL_GPL(blk_mq_request_started);

@@ -505,16 +505,10 @@ void blk_mq_start_request(struct request *rq)
*/
smp_mb__before_atomic();

- /*
-  * Mark us as started and clear complete. Complete might have been
-  * set if requeue raced with timeout, which then marked it as
-  * complete. So be sure to clear complete again when we start
-  * the request, otherwise we'll ignore the completion event.
-  */
- if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
- set_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
- if (test_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags))
- clear_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
+ WARN_ONCE(atomic_read(&rq->state) != REQ_NOT_STARTED,
+   "unexpected request state %d != %d\n",
+   atomic_read(&rq->state), REQ_NOT_STARTED);
+ atomic_set(&rq->state, REQ_STARTED);

if (q->dma_drain_size && blk_rq_bytes(rq)) {
/*
@@ -530,12 +524,14 @@ EXPORT_SYMBOL(blk_mq_start_request);
static void __blk_mq_requeue_request(struct request *rq)
{
struct request_queue *q = rq->q;
+ enum rq_state prev;

trace_block_rq_requeue(q, rq);
wbt_requeue(q->rq_wb, &rq->issue_stat);
blk_mq_sched_requeue_request(rq);

- if (test_and_clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) {
+ prev = atomic_xchg(&rq->state, REQ_NOT_STARTED);
+ if (prev != REQ_NOT_STARTED) {
if (q->dma_drain_size && blk_rq_bytes(rq))
rq->nr_phys_segments--;
}
@@ -661,17 +657,7 @@ void blk_mq_rq_timed_out(struct request *req, bool reserved)
const struct blk_mq_ops *ops = req->q->mq_ops;
enum blk_eh_timer_return ret = BLK_EH_RESET_TIMER;

- /*
-  * We know that complete is set at this point. If STARTED isn't set
-  * anymore, then the request isn't active and the "timeout" should
-  * just be ignored. This can happen due to the bitflag ordering.
-  * Timeout first checks if STARTED is set, and if it is, assumes
-  * the request is active. But if we race with completion, then
-  * we both flags will get cleared. So check here again, and ignore
-  * a timeout event with a request that isn't active.
-  */
- if (!test_bit(REQ_ATOM_STARTED, &req->atomic_flags))
- return;
+ WARN_ON_ONCE(atomic_read(&req->state) != REQ_COMPLETE);

if (ops->timeout)
ret = ops->timeout(req, reserved);
@@ -682,7 +668,7 @@ void blk_mq_rq_timed_out(struct request *req, bool reserved)
break;
case BLK_EH_RESET_TIMER:
blk_add_timer(req);
- blk_clear_rq_complete(req);
+ atomic_set(&req->state, REQ_STARTED);
break;
case BLK_EH_NOT_HANDLED:
break;
@@ -692,27 +678,24 @@ void blk_mq_rq_timed_out(struct request *req, bool reserved)
}
}

+/*
+ * Check whether or not a request has expired. This function can execute
+ * concurrently with other functions that change the request state. This can
+ * result in returning a deadline (blk_mq_timeout_data.next) that occurs
+ * before a request times out. However, this is harmless because the next
+ * call of blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &data) will
+ * yield the correct timeout, unless the same race occurs again.
+ */
static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
struct request *rq, void *priv, bool reserved)
{
struct blk_mq_timeout_data *data = priv;

- if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) {
- /*
-  * If a request wasn't started before the queue was
-  * marked dying, kill it here or it'll go unnoticed.
-  */
- if (unlikely(blk_queue_dying(rq->q))) {
- rq->errors = -EIO;
- blk_mq_end_request(rq, rq->errors);
- }
- return;
- }
-
- if (time_after_eq(jiffies, rq->deadline)) {
- if (!blk_mark_rq_complete(rq))
- blk_mq_rq_timed_out(rq, reserved);
- } else if (!data->next_set || time_after(data->next, rq->deadline)) {
+ if (time_after_eq(jiffies, rq->deadline) &&
+     !blk_mark_rq_complete_if_started(rq)) {
+ blk_mq_rq_timed_out(rq, reserved);
+ } else if ((!data->next_set || time_after(data->next, rq->deadline)) &&
+    blk_mq_request_started(rq)) {
data->next = rq->deadline;
data->next_set = 1;
}
@@ -2821,7 +2804,7 @@ static bool blk_mq_poll_hybrid_sleep(struct request_queue *q,

hrtimer_init_sleeper(&hs, current);
do {
- if (test_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags))
+ if (atomic_read(&rq->state) == REQ_COMPLETE)
break;
set_current_state(TASK_UNINTERRUPTIBLE);
hrtimer_start_expires(&hs.timer, mode);
diff --git a/block/blk-timeout.c b/block/blk-timeout.c
index a30441a200c0..9a8b44ebfb99 100644
--- a/block/blk-timeout.c
+++ b/block/blk-timeout.c
@@ -94,7 +94,7 @@ static void blk_rq_timed_out(struct request *req)
break;
case BLK_EH_RESET_TIMER:
blk_add_timer(req);
- blk_clear_rq_complete(req);
+ atomic_set(&req->state, REQ_NOT_STARTED);
break;
case BLK_EH_NOT_HANDLED:
/*
diff --git a/block/blk.h b/block/blk.h
index d1ea4bd9b9a3..8af5fe21e85f 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -115,23 +115,32 @@ void blk_account_io_done(struct request *req);
* Internal atomic flags for request handling
*/
enum rq_atomic_flags {
- REQ_ATOM_COMPLETE = 0,
- REQ_ATOM_STARTED,
REQ_ATOM_POLL_SLEPT,
};

/*
+ * Request states. Note: REQ_STARTED is only used by the blk-mq code.
+ */
+enum rq_state {
+ REQ_NOT_STARTED,
+ REQ_STARTED,
+ REQ_COMPLETE,
+};
+
+/*
* EH timer and IO completion will both attempt to 'grab' the request, make
- * sure that only one of them succeeds
+ * sure that only one of them succeeds. The return value 0 means that this
+ * function changed the request state from "not complete" into "complete".
*/
static inline int blk_mark_rq_complete(struct request *rq)
{
- return test_and_set_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
+ return atomic_xchg(&rq->state, REQ_COMPLETE) == REQ_COMPLETE;
}

-static inline void blk_clear_rq_complete(struct request *rq)
+static inline int blk_mark_rq_complete_if_started(struct request *rq)
{
- clear_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
+ return atomic_cmpxchg(&rq->state, REQ_STARTED, REQ_COMPLETE) !=
+ REQ_STARTED;
}

/*
diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 939c47df73fa..136379097131 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -672,7 +672,7 @@ static int virtscsi_tmf(struct virtio_scsi *vscsi, struct virtio_scsi_cmd *cmd)
*
* In the abort case, sc->scsi_done will do nothing, because
* the block layer must have detected a timeout and as a result
-  * REQ_ATOM_COMPLETE has been set.
+  * rq->state == REQ_COMPLETED.
*/
virtscsi_poll_requests(vscsi);

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 5a7da607ca04..b286887b095e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -142,6 +142,7 @@ struct request {

int internal_tag;

+ atomic_t state;
unsigned long atomic_flags;

/* the following two fields are internal, NEVER access directly */
--
2.12.0