linux-kernel - Re: [PATCH] ata: Add the SW NCQ support to sata

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20070627022114.455bc0d6.akpm@linux-foundation.org>
Date:	Wed, 27 Jun 2007 02:21:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kuan Luo" <kluo@...dia.com>
Cc:	<linux-kernel@...r.kernel.org>, <jeff@...zik.org>,
	"Peer Chen" <pchen@...dia.com>
Subject: Re: [PATCH] ata: Add the SW NCQ support to sata_nv for
 MCP51/MCP55/MCP61

On Wed, 27 Jun 2007 17:09:29 +0800 "Kuan Luo" <kluo@...dia.com> wrote:

> > +static void nv_swncq_qc_to_dq(struct ata_port *ap, struct
> ata_queued_cmd *qc)
> > +{
> > +	struct nv_swncq_port_priv *pp = ap->private_data;
> > +	defer_queue_t	*dq = &pp->defer_queue;
> > +	
> > +	/* queue is full */
> > +	WARN_ON((dq->rear + 1) % (ATA_MAX_QUEUE + 1) == dq->front);
> 
> >This is peculiar.  The array is sized ATA_MAX_QUEUE+1 (ie: 33) and the
> code
> >uses ATA_MAX_QUEUE+1 everywhere.
> 
> >It looks to me like the ata code was designed to queue up to 32
> elements
> >and all this code has taken that to 33.  What exactly is going on here?
> 
> 
> The code is designed to contain 32 elements. But the position of rear
> doesn't point to
> a valid element to check whether the queue is full or null. If front ==
> rear , queue is null.
>  if (rear + 1) % (ATA_MAX_QUEUE + 1) == front, queue is full.
> So the value of the array is 32 + 1.

that's the wrong way of doing a circular buffer..

It's better to allow the head and tail indices to wrap all the way through
0xffffffff and only mask them when they are actually used for subscripting.
That way:

add:
	array[head++ & (ATA_MAX_QUEUE-1)] = item;

remove:
	item = array[tail++ & (ATA_MAX_QUEUE-1)];

full:
	head - tail == ATA_MAX_QUEUE

empty:
	head == tail

number of items:
	head - tail

This requires that the array size be a power of two.


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/