Re: [PATCH v5] blkcg: allocate struct blkcg_gq outside request queue spinlock

From: Jens Axboe <axboe@kernel.dk>
To: Tahsin Erdogan <tahsin@google.com>, Tejun Heo <tj@kernel.org>
Cc: linux-block@vger.kernel.org, David Rientjes <rientjes@google.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5] blkcg: allocate struct blkcg_gq outside request queue spinlock
Date: Sat, 11 Mar 2017 15:52:33 -0700	[thread overview]
Message-ID: <e7d7aaab-3a32-61f6-44dd-3680d7f0c8db@kernel.dk> (raw)
In-Reply-To: <bb97baf4-b08a-b909-1bff-7c933b95be4e@kernel.dk>

On 03/11/2017 03:42 PM, Jens Axboe wrote:
>> @@ -185,31 +187,53 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
>>  		goto err_free_blkg;
>>  	}
>>  
>> +	if (drop_locks) {
>> +		spin_unlock_irq(q->queue_lock);
>> +		rcu_read_unlock();
>> +	}
> 
> I have a general dislike for code like that, where you conditionally
> drop locks. And this one seems even worse, since the knowledge of
> whether the locks should/could be dropped or not is embedded in the gfp
> flags.

Talked to Tejun about this as well, and we both agree that the splitting
this into separate init/alloc paths would be much cleaner. I can't
apply the current patch, sorry, it's just too ugly to live.

>> +/**
>> + * blkg_lookup_create - lookup blkg, try to create one if not there
>> + *
>> + * Performs an initial queue bypass check and then passes control to
>> + * __blkg_lookup_create().
>> + */
>> +struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
>> +				    struct request_queue *q, gfp_t gfp,
>> +				    const struct blkcg_policy *pol)
>> +{
>> +	WARN_ON_ONCE(!rcu_read_lock_held());
>> +	lockdep_assert_held(q->queue_lock);
> 
> This seems problematic, as blkcg_bio_issue_check() calls with the rcu
> read lock held.

Brain fart, that part is fine.

-- 
Jens Axboe