Re: [PATCH] block: don't dereference request after flush insertion

[Jens Axboe <axboe@kernel.dk>]

On 10/17/21 7:49 PM, Ming Lei wrote:
> On Sat, Oct 16, 2021 at 07:35:39PM -0600, Jens Axboe wrote:
>> We could have a race here, where the request gets freed before we call
>> into blk_mq_run_hw_queue(). If this happens, we cannot rely on the state
>> of the request.
>>
>> Grab the hardware context before inserting the flush.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> ---
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 2197cfbf081f..22b30a89bf3a 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -2468,9 +2468,10 @@ void blk_mq_submit_bio(struct bio *bio)
>>  	}
>>  
>>  	if (unlikely(is_flush_fua)) {
>> +		struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
>>  		/* Bypass scheduler for flush requests */
>>  		blk_insert_flush(rq);
>> -		blk_mq_run_hw_queue(rq->mq_hctx, true);
>> +		blk_mq_run_hw_queue(hctx, true);
> 
> If the request is freed before running queue, the request queue could
> be released and the hctx may be freed.

No, we still hold a queue enter ref.

-- 
Jens Axboe