linux-kernel - Re: [PATCH 3/3] blk-mq: Fix the queue freezing mechanism

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Message-ID: <56042844.60603@sandisk.com>
Date:	Thu, 24 Sep 2015 09:43:48 -0700
From:	Bart Van Assche <bart.vanassche@...disk.com>
To:	Ming Lei <tom.leiming@...il.com>
CC:	Jens Axboe <axboe@...com>, Christoph Hellwig <hch@....de>,
	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Akinobu Mita <akinobu.mita@...il.com>
Subject: Re: [PATCH 3/3] blk-mq: Fix the queue freezing mechanism

On 09/23/2015 08:23 PM, Ming Lei wrote:
> IMO, mq_freeze_depth should only be accessed in slow path, and looks
> the race just happens during the small window between increasing
> 'mq_freeze_depth' and killing the percpu counter.

Hello Ming,

My concern is that *not* checking mq_freeze_depth in the hot path can 
cause a livelock. If there is a software layer, e.g. multipathd, that 
periodically submits new commands and if these commands take time to 
process e.g. because the transport layer is unavailable, how to 
guarantee that freezing ever succeeds without checking mq_freeze_depth 
in the hot path ?

Thanks,

Bart.
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