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Subject[PATCH 1/2] blk-mq: Factor out [s]rcu synchronization
Factor out [s]rcu synchronization in blk_mq_timeout_work() into
blk_mq_timeout_sync_rcu(). This is to add another user in the future
and doesn't cause any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Bart Van Assche <Bart.VanAssche@wdc.com>
---
Hello,

We were tracking this down in the following thread

http://lkml.kernel.org/r/20180207011133.25957-1-bart.vanassche@wdc.com

but lost the reproducer in the middle and couldn't fully verify these
two patches fix the problem; however, the identified race is real and
a bug, so I think it'd be best to apply these two patches.

Given the lack of further reports on this front, I don't think it's
necessary to rush these patches. I think it'd be best to apply these
once the merge window closes. If we need to backport these to
-stable, we can tag them later.

Thanks.

block/blk-mq.c | 39 +++++++++++++++++++++++----------------
include/linux/blk-mq.h | 2 +-
2 files changed, 24 insertions(+), 17 deletions(-)

--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -876,13 +876,34 @@ static void blk_mq_check_expired(struct
time_after_eq(jiffies, deadline)) {
blk_mq_rq_update_aborted_gstate(rq, gstate);
data->nr_expired++;
- hctx->nr_expired++;
+ hctx->need_sync_rcu = true;
} else if (!data->next_set || time_after(data->next, deadline)) {
data->next = deadline;
data->next_set = 1;
}
}

+static void blk_mq_timeout_sync_rcu(struct request_queue *q)
+{
+ struct blk_mq_hw_ctx *hctx;
+ bool has_rcu = false;
+ int i;
+
+ queue_for_each_hw_ctx(q, hctx, i) {
+ if (!hctx->need_sync_rcu)
+ continue;
+
+ if (!(hctx->flags & BLK_MQ_F_BLOCKING))
+ has_rcu = true;
+ else
+ synchronize_srcu(hctx->srcu);
+
+ hctx->need_sync_rcu = false;
+ }
+ if (has_rcu)
+ synchronize_rcu();
+}
+
static void blk_mq_terminate_expired(struct blk_mq_hw_ctx *hctx,
struct request *rq, void *priv, bool reserved)
{
@@ -930,27 +951,13 @@ static void blk_mq_timeout_work(struct w
blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &data);

if (data.nr_expired) {
- bool has_rcu = false;
-
/*
* Wait till everyone sees ->aborted_gstate. The
* sequential waits for SRCUs aren't ideal. If this ever
* becomes a problem, we can add per-hw_ctx rcu_head and
* wait in parallel.
*/
- queue_for_each_hw_ctx(q, hctx, i) {
- if (!hctx->nr_expired)
- continue;
-
- if (!(hctx->flags & BLK_MQ_F_BLOCKING))
- has_rcu = true;
- else
- synchronize_srcu(hctx->srcu);
-
- hctx->nr_expired = 0;
- }
- if (has_rcu)
- synchronize_rcu();
+ blk_mq_timeout_sync_rcu(q);

/* terminate the ones we won */
blk_mq_queue_tag_busy_iter(q, blk_mq_terminate_expired, NULL);
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -51,7 +51,7 @@ struct blk_mq_hw_ctx {
unsigned int queue_num;

atomic_t nr_active;
- unsigned int nr_expired;
+ bool need_sync_rcu;

struct hlist_node cpuhp_dead;
struct kobject kobj;
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