On Tue, Jan 23, 2018 at 08:37:26AM -0800, Bart Van Assche wrote: > 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 Could you explain where to call call_rcu()? call_rcu() can't be used in IO path at all. > 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. -- Ming
