Re: [PATCH 2/5] bfq: protect q->blkg_list iteration in bfq_end_wr_async() with blkcg_mutex

[yu kuai]

Hi,

在 2026/6/5 1:31, Nilay Shroff 写道:
> On 6/3/26 6:57 PM, Yu Kuai wrote:
>> bfq_end_wr_async() iterates q->blkg_list while only holding bfqd->lock,
>> but not blkcg_mutex. This can race with blkg_free_workfn() that removes
>> blkgs from the list while holding blkcg_mutex.
>>
>> Add blkcg_mutex protection in bfq_end_wr() before taking bfqd->lock to
>> ensure proper synchronization when iterating q->blkg_list.
>>
>> Signed-off-by: Yu Kuai <yukuai@xxxxxxx>
>> ---
>>   block/bfq-cgroup.c  | 3 ++-
>>   block/bfq-iosched.c | 6 ++++++
>>   2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
>> index 37ab70930c8d..f765e767d36a 100644
>> --- a/block/bfq-cgroup.c
>> +++ b/block/bfq-cgroup.c
>> @@ -939,11 +939,12 @@ void bfq_end_wr_async(struct bfq_data *bfqd)
>>       struct blkcg_gq *blkg;
>>         list_for_each_entry(blkg, &bfqd->queue->blkg_list, q_node) {
>>           struct bfq_group *bfqg = blkg_to_bfqg(blkg);
>>   -        bfq_end_wr_async_queues(bfqd, bfqg);
>> +        if (bfqg)
>> +            bfq_end_wr_async_queues(bfqd, bfqg);
>>       }
>>       bfq_end_wr_async_queues(bfqd, bfqd->root_group);
>>   }
>>     static int bfq_io_show_weight_legacy(struct seq_file *sf, void *v)
>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>> index 141c602d5e85..42ccfd0c6140 100644
>> --- a/block/bfq-iosched.c
>> +++ b/block/bfq-iosched.c
>> @@ -2643,10 +2643,13 @@ void bfq_end_wr_async_queues(struct bfq_data 
>> *bfqd,
>>   static void bfq_end_wr(struct bfq_data *bfqd)
>>   {
>>       struct bfq_queue *bfqq;
>>       int i;
>>   +#ifdef CONFIG_BFQ_GROUP_IOSCHED
>> +    mutex_lock(&bfqd->queue->blkcg_mutex);
>> +#endif
>>       spin_lock_irq(&bfqd->lock);
>>         for (i = 0; i < bfqd->num_actuators; i++) {
>>           list_for_each_entry(bfqq, &bfqd->active_list[i], bfqq_list)
>>               bfq_bfqq_end_wr(bfqq);
>> @@ -2654,10 +2657,13 @@ static void bfq_end_wr(struct bfq_data *bfqd)
>>       list_for_each_entry(bfqq, &bfqd->idle_list, bfqq_list)
>>           bfq_bfqq_end_wr(bfqq);
>>       bfq_end_wr_async(bfqd);
>>         spin_unlock_irq(&bfqd->lock);
>> +#ifdef CONFIG_BFQ_GROUP_IOSCHED
>> +    mutex_unlock(&bfqd->queue->blkcg_mutex);
>> +#endif
>>   }
>
> The above change protects the q->blkg_list iteration in 
> bfq_end_wr_async()
> against list removal in blkg_free_workfn(). However the blkg insertion in
> blkg_create() still doesn't use q->blkcg_mutex and so list traversal in
> bfq_end_wr_async() may still race with blkg_create().
>
> So I think we may also need to protect blkg insert in blkg_create() using
> q->blkcg_mutex.

Yes, this is done in another huge patchset, because currently blkg_create()
is protected by queue_lock and can be called under rcu, code refactor will
be required.

I'll send the first set soon.

>
> Thanks,
> --Nilay

-- 
Thanks,
Kuai