Currently, hctx attributes (nr_tags, nr_reserved_tags, and cpu_list)
are protected using `q->sysfs_lock`. However, these attributes can be
updated in multiple scenarios:
- During the driver's probe method.
- When updating nr_hw_queues.
- When writing to the sysfs attribute nr_requests,
which can modify nr_tags.
The nr_requests attribute is already protected using q->elevator_lock,
but none of the update paths actually use q->sysfs_lock to protect hctx
attributes. So to ensure proper synchronization, replace q->sysfs_lock
with q->elevator_lock when reading hctx attributes through sysfs.
Additionally, blk_mq_update_nr_hw_queues allocates and updates hctx.
The allocation of hctx is protected using q->elevator_lock, however,
updating hctx params happens without any protection, so safeguard hctx
param update path by also using q->elevator_lock.
Signed-off-by: Nilay Shroff <nilay@xxxxxxxxxxxxx>
---
Please note that this patch was unit tested against blktests and
quick xfstest with lockdep enabled.
This patch should go on top of the previous patchset:
https://lore.kernel.org/all/20250304102551.2533767-1-nilay@xxxxxxxxxxxxx/
---
block/blk-mq-sysfs.c | 4 ++--
block/blk-mq.c | 4 ++++
include/linux/blkdev.h | 14 ++++++++------
3 files changed, 14 insertions(+), 8 deletions(-)
diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 3feeeccf8a99..24656980f443 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -61,9 +61,9 @@ static ssize_t blk_mq_hw_sysfs_show(struct kobject *kobj,
if (!entry->show)
return -EIO;
- mutex_lock(&q->sysfs_lock);
+ mutex_lock(&q->elevator_lock);
res = entry->show(hctx, page);
- mutex_unlock(&q->sysfs_lock);
+ mutex_unlock(&q->elevator_lock);
return res;
}
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 5a2d63927525..b9550a127c8e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4094,6 +4094,8 @@ static void blk_mq_map_swqueue(struct request_queue *q)
struct blk_mq_ctx *ctx;
struct blk_mq_tag_set *set = q->tag_set;
+ mutex_lock(&q->elevator_lock);
+
queue_for_each_hw_ctx(q, hctx, i) {
cpumask_clear(hctx->cpumask);
hctx->nr_ctx = 0;
@@ -4198,6 +4200,8 @@ static void blk_mq_map_swqueue(struct request_queue *q)
hctx->next_cpu = blk_mq_first_mapped_cpu(hctx);
hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
}
+
+ mutex_unlock(&q->elevator_lock);
}
/*
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 4daef0408683..b6f893e1741b 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -563,12 +563,14 @@ struct request_queue {
struct list_head flush_list;
/*
- * Protects against I/O scheduler switching, particularly when
- * updating q->elevator. Since the elevator update code path may
- * also modify q->nr_requests and wbt latency, this lock also
- * protects the sysfs attributes nr_requests and wbt_lat_usec.
- * To ensure proper locking order during an elevator update, first
- * freeze the queue, then acquire ->elevator_lock.
+ * Protects against I/O scheduler switching, particularly when updating
+ * q->elevator. Since the elevator update code path may also modify q->
+ * nr_requests and wbt latency, this lock also protects the sysfs attrs
+ * nr_requests and wbt_lat_usec. Additionally the nr_hw_queues update may
+ * modify hctx tags, reserved-tags and cpumask, so this lock also helps
+ * protect the hctx attrs.
+ * To ensure proper locking order during an elevator or nr_hw_queue
+ * update, first freeze the queue, then acquire ->elevator_lock.
*/
struct mutex elevator_lock;
--
2.47.1
