Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme

[tj@xxxxxxxxxx]

Hello, Peter.

On Thu, Dec 14, 2017 at 09:20:42PM +0100, Peter Zijlstra wrote:
> But now that I look at this again, TJ, why can't the below happen?
> 
> 	write_seqlock_begin();
> 	blk_mq_rq_update_state(rq, IN_FLIGHT);
> 	blk_add_timer(rq);
> 	<timer-irq>
> 		read_seqcount_begin()
> 			while (seq & 1)
> 				cpurelax();
> 		// life-lock
> 	</timer-irq>
> 	write_seqlock_end();

Ah, you're right.  For both gstate_seq and aborted_gstate_sync, we can
push all synchronization to the timeout side - ie. gstate_seq read can
yield, pause or synchronize_rcu and hte aborted_gstate_sync can
disable irq around update.

Thanks.

-- 
tejun