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Subject: Re: [PATCH -next v2 8/9] block: fix null-pointer dereference in
 ioc_pd_init
To:     Tejun Heo <tj@kernel.org>, Yu Kuai <yukuai1@huaweicloud.com>
Cc:     Li Nan <linan122@huawei.com>, josef@toxicpanda.com,
        axboe@kernel.dk, cgroups@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
References: <20221130132156.2836184-1-linan122@huawei.com>
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From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <431dcb3f-4572-7fd0-9e5d-90b6c34d577c@huaweicloud.com>
Date:   Thu, 1 Dec 2022 18:23:16 +0800
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Hi,

?? 2022/12/01 18:11, Tejun Heo д??:
> On Thu, Dec 01, 2022 at 10:12:13AM +0800, Yu Kuai wrote:
>> 1) By mentioning that "del_gendisk() is quiescing the queue", do you
>> suggest to add rcu_read_lock()? This seems wrong because blk_iocost_init
>> requires memory allocation.
> 
> Quiescing uses SRCU so that should be fine but I'm not sure whether this is
> the right one to piggyback on. Jens should have a better idea.
> 
> Thanks.
> 

Currently SRCU is used if BLK_MQ_F_BLOCKING set, otherwise RCU is used.

dispatch:
#define __blk_mq_run_dispatch_ops(q, check_sleep, dispatch_ops) \
do {                                                            \
         if ((q)->tag_set->flags & BLK_MQ_F_BLOCKING) {          \
                 int srcu_idx;                                   \
                                                                 \
                 might_sleep_if(check_sleep);                    \
                 srcu_idx = srcu_read_lock((q)->tag_set->srcu);  \
                 (dispatch_ops);                                 \
                 srcu_read_unlock((q)->tag_set->srcu, srcu_idx); \
         } else {                                                \
                 rcu_read_lock();                                \
                 (dispatch_ops);                                 \
                 rcu_read_unlock();                              \
         }                                                       \
} while (0)

quiesce:
void blk_mq_wait_quiesce_done(struct blk_mq_tag_set *set)
{
         if (set->flags & BLK_MQ_F_BLOCKING)
                 synchronize_srcu(set->srcu);
         else
                 synchronize_rcu();
}

Thanks,
Kuai