linux-kernel - Re: [PATCH 01/14] pch_uart: drop double zeroing

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Date:   Sun, 20 Sep 2020 14:47:11 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
cc:     kernel-janitors@...r.kernel.org, Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/14] pch_uart: drop double zeroing



On Sun, 20 Sep 2020, Greg Kroah-Hartman wrote:

> On Sun, Sep 20, 2020 at 01:26:13PM +0200, Julia Lawall wrote:
> > sg_init_table zeroes its first argument, so the allocation of that argument
> > doesn't have to.
> >
> > the semantic patch that makes this change is as follows:
> > (http://coccinelle.lip6.fr/)
> >
> > // <smpl>
> > @@
> > expression x,n,flags;
> > @@
> >
> > x =
> > - kcalloc
> > + kmalloc_array
> >   (n,sizeof(struct scatterlist),flags)
> > ...
> > sg_init_table(x,n)
> > // </smpl>
> >
> > Signed-off-by: Julia Lawall <Julia.Lawall@...ia.fr>
>
> It inits the first entry in the array, but what about all of the other
> ones?  Is that "safe" to have uninitialized data in them like your
> change causes to happen?

Sorry, I don't follow.  The complete code is:

        priv->sg_tx_p = kcalloc(num, sizeof(struct scatterlist), GFP_ATOMIC);
        if (!priv->sg_tx_p) {
		dev_err(priv->port.dev, "%s:kzalloc Failed\n", __func__);
                return 0;
	}

	sg_init_table(priv->sg_tx_p, num); /* Initialize SG table */

and the definition of sg_init_table is:

void sg_init_table(struct scatterlist *sgl, unsigned int nents)
{
	memset(sgl, 0, sizeof(*sgl) * nents);
	sg_init_marker(sgl, nents);
}

It looks to me like it zeroes all of the elements?  The same file does
contain a call:

sg_init_table(&priv->sg_rx, 1);

But that's not the one associated with the patch.

julia