[dm-devel] [PATCH 1/5] blk-mq: introduce BLK_STS_DEV_RESOURCE

[Bart Van Assche]

On 01/23/18 08:26, Ming Lei wrote:
> On Tue, Jan 23, 2018 at 08:17:02AM -0800, Bart Van Assche wrote:
>> On 01/22/18 16:57, Ming Lei wrote:
>>> Even though RCU lock is held during dispatch, preemption or interrupt
>>> can happen too, so it is simply wrong to depend on the timing to make
>>> sure __blk_mq_run_hw_queue() can see the request in this situation.
>>
>> It is very unlikely that this race will ever be hit because that race exists
>> for less than one microsecond and the smallest timeout that can be specified
>> for delayed queue rerunning is one millisecond. Let's address this race if
>> anyone ever finds a way to hit it.
> 
> Please don't depend on the timing which is often fragile, as we can make it
> correct in a generic approach. Also we should avoid to make every driver to
> follow this way for dealing with most of STS_RESOURCE, right?

Wouldn't it be better to fix that race without changing the block layer 
API, e.g. by using call_rcu() for delayed queue runs? As you know 
call_rcu() will only call the specified function after a grace period. 
Since pushing back requests onto the dispatch list happens with the RCU 
lock held using call_rcu() for delayed queue runs would be sufficient to 
address this race.

Bart.