On 8/14/25 6:14 PM, Ming Lei wrote:
> On Thu, Aug 14, 2025 at 01:54:59PM +0530, Nilay Shroff wrote:
>> A recent lockdep[1] splat observed while running blktest block/005
>> reveals a potential deadlock caused by the cpu_hotplug_lock dependency
>> on ->freeze_lock. This dependency was introduced by commit 033b667a823e
>> ("block: blk-rq-qos: guard rq-qos helpers by static key").
>>
>> That change added a static key to avoid fetching q->rq_qos when
>> neither blk-wbt nor blk-iolatency is configured. The static key
>> dynamically patches kernel text to a NOP when disabled, eliminating
>> overhead of fetching q->rq_qos in the I/O hot path. However, enabling
>> a static key at runtime requires acquiring both cpu_hotplug_lock and
>> jump_label_mutex. When this happens after the queue has already been
>> frozen (i.e., while holding ->freeze_lock), it creates a locking
>> dependency from cpu_hotplug_lock to ->freeze_lock, which leads to a
>> potential deadlock reported by lockdep [1].
>>
>> To resolve this, replace the static key mechanism with q->queue_flags:
>> QUEUE_FLAG_QOS_ENABLED. This flag is evaluated in the fast path before
>> accessing q->rq_qos. If the flag is set, we proceed to fetch q->rq_qos;
>> otherwise, the access is skipped.
>>
>> Since q->queue_flags is commonly accessed in IO hotpath and resides in
>> the first cacheline of struct request_queue, checking it imposes minimal
>> overhead while eliminating the deadlock risk.
>>
>> This change avoids the lockdep splat without introducing performance
>> regressions.
>>
>> [1] https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/
>>
>> Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@xxxxxxx>
>> Closes: https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/
>> Fixes: 033b667a823e ("block: blk-rq-qos: guard rq-qos helpers by static key")
>> Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@xxxxxxx>
>> Signed-off-by: Nilay Shroff <nilay@xxxxxxxxxxxxx>
>> ---
>> block/blk-mq-debugfs.c | 1 +
>> block/blk-rq-qos.c | 9 ++++---
>> block/blk-rq-qos.h | 54 ++++++++++++++++++++++++------------------
>> include/linux/blkdev.h | 1 +
>> 4 files changed, 37 insertions(+), 28 deletions(-)
>>
>> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
>> index 7ed3e71f2fc0..32c65efdda46 100644
>> --- a/block/blk-mq-debugfs.c
>> +++ b/block/blk-mq-debugfs.c
>> @@ -95,6 +95,7 @@ static const char *const blk_queue_flag_name[] = {
>> QUEUE_FLAG_NAME(SQ_SCHED),
>> QUEUE_FLAG_NAME(DISABLE_WBT_DEF),
>> QUEUE_FLAG_NAME(NO_ELV_SWITCH),
>> + QUEUE_FLAG_NAME(QOS_ENABLED),
>> };
>> #undef QUEUE_FLAG_NAME
>>
>> diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
>> index b1e24bb85ad2..654478dfbc20 100644
>> --- a/block/blk-rq-qos.c
>> +++ b/block/blk-rq-qos.c
>> @@ -2,8 +2,6 @@
>>
>> #include "blk-rq-qos.h"
>>
>> -__read_mostly DEFINE_STATIC_KEY_FALSE(block_rq_qos);
>> -
>> /*
>> * Increment 'v', if 'v' is below 'below'. Returns true if we succeeded,
>> * false if 'v' + 1 would be bigger than 'below'.
>> @@ -319,8 +317,8 @@ void rq_qos_exit(struct request_queue *q)
>> struct rq_qos *rqos = q->rq_qos;
>> q->rq_qos = rqos->next;
>> rqos->ops->exit(rqos);
>> - static_branch_dec(&block_rq_qos);
>> }
>> + blk_queue_flag_clear(QUEUE_FLAG_QOS_ENABLED, q);
>> mutex_unlock(&q->rq_qos_mutex);
>> }
>>
>> @@ -346,7 +344,7 @@ int rq_qos_add(struct rq_qos *rqos, struct gendisk *disk, enum rq_qos_id id,
>> goto ebusy;
>> rqos->next = q->rq_qos;
>> q->rq_qos = rqos;
>> - static_branch_inc(&block_rq_qos);
>> + blk_queue_flag_set(QUEUE_FLAG_QOS_ENABLED, q);
>
> One stupid question: can we simply move static_branch_inc(&block_rq_qos)
> out of queue freeze in rq_qos_add()?
>
> What matters is just the 1st static_branch_inc() which switches the counter
> from 0 to 1, when blk_mq_freeze_queue() guarantees that all in-progress code
> paths observe q->rq_qos as NULL. That means static_branch_inc(&block_rq_qos)
> needn't queue freeze protection.
>
I thought about it earlier but that won't work because we have
code paths freezing queue before it reaches upto rq_qos_add(),
For instance:
We have following code paths from where we invoke
rq_qos_add() APIs with queue already frozen:
ioc_qos_write()
-> blkg_conf_open_bdev_frozen() => freezes queue
-> blk_iocost_init()
-> rq_qos_add()
queue_wb_lat_store() => freezes queue
-> wbt_init()
-> rq_qos_add()
And yes we do have code path leading to rq_qos_exit()
which freezes queue first:
__del_gendisk() => freezes queue
-> rq_qos_exit()
And similarly for rq_qos_delete():
ioc_qos_write()
-> blkg_conf_open_bdev_frozen() => freezes queue
-> blk_iocost_init()
-> rq_qos_del()
So even if we move static_branch_inc() out of freeze queue
in rq_qos_add(), it'd still cause the observed lockdep
splat.
Thanks,
--Nilay
