linux-kernel - Re: [PATCH v2 1/4] Wrap crc_t10dif function all to use crypto transform framework

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Message-ID: <1367268030.27102.172.camel@schen9-DESK>
Date:	Mon, 29 Apr 2013 13:40:30 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Jim Kukunas <james.t.kukunas@...ux.intel.com>,
	Keith Busch <keith.busch@...el.com>,
	Erdinc Ozturk <erdinc.ozturk@...el.com>,
	Vinodh Gopal <vinodh.gopal@...el.com>,
	James Guilford <james.guilford@...el.com>,
	Wajdi Feghali <wajdi.k.feghali@...el.com>,
	Jussi Kivilinna <jussi.kivilinna@....fi>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH v2 1/4] Wrap crc_t10dif function all to use crypto
 transform framework

On Sun, 2013-04-28 at 08:11 +0800, Herbert Xu wrote:
> On Fri, Apr 26, 2013 at 09:44:17AM -0700, Tim Chen wrote:
> >
> > +	old_tfm = crct10dif_tfm;
> > +	crc_t10dif_newalg = true;
> > +	/* make sure new alg flag is turned on before starting to switch tfm */
> > +	mb();
> > +
> > +	new_tfm = crypto_alloc_shash("crct10dif", 0, 0);
> > +	if (IS_ERR(new_tfm))
> > +		goto done;
> > +
> > +	if (old_tfm)
> > +		if (crypto_tfm_alg_priority(&old_tfm->base) >=
> > +		    crypto_tfm_alg_priority(&new_tfm->base)) {
> > +			crypto_free_shash(new_tfm);
> > +			goto done;
> > +		}
> > +	crct10dif_tfm = new_tfm;
> > +	/* make sure update to tfm pointer is completed */
> > +	mb();
> > +	crypto_free_shash(old_tfm);
> 
> This is not safe at all.  You'd need to use something like RCU.
> 
> However, I think this is an overkill.  Initialising it once should
> be enough.  If someone really wanted to change things at run-time,
> they could always build this as a module and unload/reload it.
> 

If I allocate the transform under the mod init instead, how can I make
sure that the fast version is already registered if I have it compiled
in?  It is not clear to me how that's done looking at the libcrc32c
code.  

Thanks.

Tim

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