linux-kernel - Re: [PATCH 0/8] Drop host_lock around LLD SHT->queuecommand() caller

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Date:	Fri, 17 Sep 2010 12:01:52 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Vasu Dev <vasu.dev@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	James Bottomley <James.Bottomley@...e.de>,
	Mike Christie <michaelc@...wisc.edu>,
	James Smart <james.smart@...lex.com>,
	Andrew Vasquez <andrew.vasquez@...gic.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Hannes Reinecke <hare@...e.de>,
	Joe Eykholt <jeykholt@...co.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 0/8] Drop host_lock around LLD SHT->queuecommand()
 caller

On Fri, 2010-09-17 at 14:26 -0400, Jeff Garzik wrote:

> 
> But that raises the familiar tale of:  using multiple atomics (w/ their 
> locked instructions) may cost more than a spinlock.
> 

As brought up by James earlier, the scsi_host lock was used in so many
places that getting rid of it here to reduce contention for it will be a
net win, even with the cost of the atomics.  And for most LLDs that
don't need the serial number, we can just initialize it to zero for
those and have no need to increment it (thus also avoiding the atomics'
cost).

Tim

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