Re: [PATCH 4/7] md/raid10: raid10_write_request() drops the barrier before calling

[Abd-Alrhman Masalkhi <abd.masalkhi@gmail.com>]

Hi Kaui,

On Wed, Jun 24, 2026 at 15:23 +0800, yu kuai wrote:
> Hi,
>
> 在 2026/6/23 15:24, Abd-Alrhman Masalkhi 写道:
>> bio_submit_split_bioset() and reacquires it afterwards. This is
>> unnecessary because bio_submit_split_bioset() does not require
>> releasing the barrier protection.
>
> Why is this safe?
>
> It's possible plug is not enabled in this context, and the allocated new
> aplit bio will be submitted and enter wait_barrier() again. wait_barrier()
> is not supposed to be called recursively. Otherwise deadlock can happend:
>
> t1: sync_thread grab barrier and waiting for nr_pending to be 0 from raise_barrier()
> t2: wait_barrier() first inc nr_pending, then recursive wait_barrier() waiting
> for barrier to be released.
>

raid10_write_request() is only called from the bio submission path and
not from raid10d thread, so we do not re-enter it while already holding
barrier. Therefore, the split bio submitted by bio_submit_split_bioset()
cannot recurse back into wait_barrier() from the same execution path.

The situation is different for reads, where raid10_read_request() will
be called both from the submission path and from raid10d. There,
dropping and reacquiring the barrier protection remains necessary.

>>
>> Remove the redundant allow_barrier()/wait_barrier() pair around
>> bio_submit_split_bioset().
>>
>> Signed-off-by: Abd-Alrhman Masalkhi <abd.masalkhi@gmail.com>
>> ---
>>   drivers/md/raid10.c | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
>> index 840f0446c231..4bc1d5553ec7 100644
>> --- a/drivers/md/raid10.c
>> +++ b/drivers/md/raid10.c
>> @@ -1493,10 +1493,8 @@ static bool raid10_write_request(struct mddev *mddev, struct bio *bio,
>>   		if (atomic)
>>   			goto err_handle;
>>   
>> -		allow_barrier(conf);
>>   		bio = bio_submit_split_bioset(bio, r10_bio->sectors,
>>   					      &conf->bio_split);
>> -		wait_barrier(conf, false);
>>   		if (!bio) {
>>   			set_bit(R10BIO_Returned, &r10_bio->state);
>>   			goto err_handle;
>
> -- 
> Thanks,
> Kuai

-- 
Best Regards,
Abd-Alrhman