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blk-mq: fix potential deadlock while nr_requests grown 

Allocate and free sched_tags while queue is freezed can deadlock[1],
this is a long term problem, hence allocate memory before freezing
queue and free memory after queue is unfreezed.

[1] https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@linux.ibm.com/
Fixes: e3a2b3f931 ("blk-mq: allow changing of queue depth through sysfs")

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Yu Kuai 2025-09-10 16:04:43 +08:00 committed by Jens Axboe
parent 6293e336f6
commit b86433721f
3 changed files with 34 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

22
block/blk-mq.c
View File

@ -4926,11 +4926,13 @@ void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
}
EXPORT_SYMBOL(blk_mq_free_tag_set);
int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
struct elevator_tags *blk_mq_update_nr_requests(struct request_queue *q,
struct elevator_tags *et,
unsigned int nr)
{
struct blk_mq_tag_set *set = q->tag_set;
struct elevator_tags *old_et = NULL;
struct blk_mq_hw_ctx *hctx;
int ret = 0;
unsigned long i;
blk_mq_quiesce_queue(q);
@ -4965,24 +4967,18 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
}
} else {
/* Non-shared sched tags, and tags grow */
queue_for_each_hw_ctx(q, hctx, i) {
if (!hctx->sched_tags)
continue;
ret = blk_mq_tag_update_depth(hctx, &hctx->sched_tags,
nr);
if (ret)
goto out;
}
queue_for_each_hw_ctx(q, hctx, i)
hctx->sched_tags = et->tags[i];
old_et = q->elevator->et;
q->elevator->et = et;
}
q->nr_requests = nr;
if (q->elevator && q->elevator->type->ops.depth_updated)
q->elevator->type->ops.depth_updated(q);
out:
blk_mq_unquiesce_queue(q);
return ret;
return old_et;
}
/*

5
block/blk-mq.h
View File

@ -6,6 +6,7 @@
#include "blk-stat.h"
struct blk_mq_tag_set;
struct elevator_tags;
struct blk_mq_ctxs {
struct kobject kobj;
@ -45,7 +46,9 @@ void blk_mq_submit_bio(struct bio *bio);
int blk_mq_poll(struct request_queue *q, blk_qc_t cookie, struct io_comp_batch *iob,
unsigned int flags);
void blk_mq_exit_queue(struct request_queue *q);
int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr);
struct elevator_tags *blk_mq_update_nr_requests(struct request_queue *q,
struct elevator_tags *tags,
unsigned int nr);
void blk_mq_wake_waiters(struct request_queue *q);
bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *,
bool);

29
block/blk-sysfs.c
View File

@ -64,11 +64,12 @@ static ssize_t queue_requests_show(struct gendisk *disk, char *page)
static ssize_t
queue_requests_store(struct gendisk *disk, const char *page, size_t count)
{
unsigned long nr;
int ret, err;
unsigned int memflags;
struct request_queue *q = disk->queue;
struct blk_mq_tag_set *set = q->tag_set;
struct elevator_tags *et = NULL;
unsigned int memflags;
unsigned long nr;
int ret;
ret = queue_var_store(&nr, page, count);
if (ret < 0)
@ -99,16 +100,28 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
goto unlock;
}
if (!blk_mq_is_shared_tags(set->flags) && q->elevator &&
nr > q->elevator->et->nr_requests) {
/*
* Tags will grow, allocate memory before freezing queue to
* prevent deadlock.
*/
et = blk_mq_alloc_sched_tags(set, q->nr_hw_queues, nr);
if (!et) {
ret = -ENOMEM;
goto unlock;
}
}
memflags = blk_mq_freeze_queue(q);
mutex_lock(&q->elevator_lock);
err = blk_mq_update_nr_requests(disk->queue, nr);
if (err)
ret = err;
et = blk_mq_update_nr_requests(q, et, nr);
mutex_unlock(&q->elevator_lock);
blk_mq_unfreeze_queue(q, memflags);
if (et)
blk_mq_free_sched_tags(et, set);
unlock:
up_write(&set->update_nr_hwq_lock);
return ret;