linux-kernel - Re: [PATCH 1/8] scsi: Drop struct Scsi_Host->host

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Date:	Fri, 17 Sep 2010 10:49:59 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	James Bottomley <James.Bottomley@...e.de>,
	Andi Kleen <ak@...ux.intel.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Vasu Dev <vasu.dev@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	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 1/8] scsi: Drop struct Scsi_Host->host_lock around
 SHT->queuecommand()

On Fri, 2010-09-17 at 09:41 -0700, Nicholas A. Bellinger wrote:

> > > 
> > > What I was actually thinking of for the atomic is that we'd let it range
> > > [1..INT_MAX] so a zero was an indicator of no use of this.  Then the
> > > actual code could become
> > > 
> > > if (atomic_read(x)) {
> > > 	do {
> > > 		y = atomic_add_return(1, x);
> > > 	} while (y == 0);
> > > }
> > 
> > The conversion of struct scsi_cmnd->serial_number to atomic_t and the
> > above code for scsi_cmd_get_serial() sounds perfectly reasonable to me.
> > 
> 
> Actually, that should be the conversion of struct
> Scsi_Host->cmd_serial_number to an atomic_t.   AFAICT there is no reason
> for struct scsi_cmnd->serial_number needing to be an atomic_t.

Just want to verify the hidden assumption we have here when the atomic
int Scsi_Host->cmd_serial_number counter overflow after increment. The
counter itself then becomes negative.  We are assuming that when we do
type conversion back to unsigned long scsi_cmnd->serial_number, we will
get the right thing.  

So for 32-bit int, we expect if we start with 0x7fffffff in hex and the
expected sequence will be

	2147483647 (int) -> 2147483647 (unsigned long)  [0x7fffffff]
	+1
	-2147483648 (int) -> 2147483648 (unsigned long) [0x80000000]
	+1
	-2147483647 (int) -> 2147483649 (unsigned long) [0x80000001]
	
If there is architecture where the above assumption is not true (which
I'm not aware of but just checking), then we should manually wrap the
atomic counter to 1 when counter overflow.  

Tim
		
		

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