Re: [PATCH 2/8] blk-mq: protect completion path with RCU

[tj@xxxxxxxxxx]

Hello, Bart.

On Tue, Jan 09, 2018 at 04:12:40PM +0000, Bart Van Assche wrote:
> I'm concerned about the additional CPU cycles needed for the new blk_mq_map_queue()
> call, although I know this call is cheap. Would the timeout code really get that

So, if that is really a concern, let's cache that mapping instead of
changing synchronization rules for that.

> much more complicated if the hctx_lock() and hctx_unlock() calls would be changed
> into rcu_read_lock() and rcu_read_unlock() calls? Would it be sufficient if
> "if (has_rcu) synchronize_rcu();" would be changed into "synchronize_rcu();" in
> blk_mq_timeout_work()?

Code-wise, it won't be too much extra code but I think diverging the
sync methods between issue and completion paths is more fragile and
likely to invite confusions and mistakes in the future.  We have the
normal path (issue&completion) synchronizing against the exception
path (timeout).  I think it's best to keep the sync constructs aligned
with that conceptual picture.

Thanks.

-- 
tejun