netdev - Re: [PATCH 2/3] cnic: Add CNIC driver.

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Date:	Fri, 23 May 2008 17:42:35 -0700
From:	"Michael Chan" <mchan@...adcom.com>
To:	"Roland Dreier" <rdreier@...co.com>
cc:	"David Miller" <davem@...emloft.net>, michaelc@...wisc.edu,
	anilgv@...adcom.com, netdev <netdev@...r.kernel.org>,
	linux-scsi@...r.kernel.org, open-iscsi@...glegroups.com
Subject: Re: [PATCH 2/3] cnic: Add CNIC driver.

On Fri, 2008-05-23 at 13:14 -0700, Roland Dreier wrote:
>  > +		/* Tell compiler that status_blk fields can change. */
>  > +		barrier();
>  > +		if (status_idx != sblk->status_idx) {
> 
> Is a compiler barrier sufficient here?  It seems an out-of-order CPU
> could still mess things up, so an rmb() or smp_rmb() would be required.
> 

The purpose of this compiler barrier is to make sure that the compiled
code will get the sblk->status_idx from memory which may change if the
chip DMAs a new status_idx.

This code is inside a while loop, so it is possible that the compiler
can optimize it to not load status_idx from memory.

Thanks.

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