linux-kernel - Re: [PATCH 09/14] scsi: fix the {host,target,device}

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Message-ID: <20140710060617.GB7675@lst.de>
Date:	Thu, 10 Jul 2014 08:06:17 +0200
From:	Christoph Hellwig <hch@....de>
To:	Hannes Reinecke <hare@...e.de>
Cc:	Christoph Hellwig <hch@....de>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Jens Axboe <axboe@...nel.dk>,
	Bart Van Assche <bvanassche@...ionio.com>,
	Robert Elliott <Elliott@...com>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/14] scsi: fix the {host,target,device}_blocked
	counter mess

On Wed, Jul 09, 2014 at 01:12:17PM +0200, Hannes Reinecke wrote:
> Hmm. I guess there is a race window between
> atomic_read() and atomic_set().
> Doesn't this cause issues when someone calls atomic_set() just before the 
> call to atomic_read?

There is a race window just _after_ the atomic_read, but it's harmless.
The whole _blocked scheme is a backoff to avoid resubmitting I/O all
the time when the HBA or target returned a busy status.  If we race
an incorrectly reset it we will submit I/O and just get a busy indicator
again.

On the other hand doing the atomic_set all the time introduces three atomic
in the I/O completion part that are entirely pointless most of the time.

I guess I should add something like this as a comment to the code..

Note that the old code didn't use any sort of synchronization either.

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