linux-kernel - Re: [PATCH] jffs2: Don't add summary entry when MTD write fails

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Message-ID: <OFE18455CD.1A74662E-ONC1257F64.005F2536-C1257F64.0061DD82@rohde-schwarz.com>
Date:	Thu, 25 Feb 2016 18:48:56 +0100
From:	Thomas.Betker@...de-schwarz.com
To:	dwmw2@...radead.org
Cc:	linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
	Thomas Betker <thomas.betker@...enet.de>
Subject: Re: [PATCH] jffs2: Don't add summary entry when MTD write fails

Hello David:

> > +       int ret;
> > +
> > +       ret = mtd_writev(c->mtd, vecs, count, to, retlen);
> > +
> >         if (!jffs2_is_writebuffered(c)) {
> >                 if (jffs2_sum_active()) {
> >                         int res;
> > +
> > +                       if (ret ||
> > +                           *retlen != iov_length((struct iovec *)
> vecs, count))
> > +                               return ret;
> > +
> >                         res = jffs2_sum_add_kvec(c, vecs, count, 
> (uint32_t) to);
> >                         if (res) {
> >                                 return res;
> 
> OK... but perhaps we can dispense with the separate 'ret' and 'res'
> variables and the rats nest of conditions, and do something like:
> 
>    int ret;
> 
>    ret = mtd_writev(…);
> 
>    if (!ret && *retlen == iov_length(…) &&
>        !jffs2_is_writebuffered(c) && jffs2_sum_active()) 
>           ret = jffs2_sum_add_kvec(…);
>    
>    return ret;

While the logic is the same, will the compiler generate the same code? 
When CONFIG_JFFS2_SUMMARY is not set, "if (jffs2_sum_active())" means "if 
(0)", and I would assume that the compiler removes the whole clause, "if" 
and all. However, I am not sure what happens with "if (!ret && whatever && 
0)".

That's why I was taking pains to keep the original control flow intact, 
even if it's a rat's nest (it is). If I remember correctly, 
jffs2_flash_direct_writev() is called quite often, and I didn't want 
performance to suffer. I may be completely wrong here, of course, but then 
why wasn't the original source code "if (!jffs2_is_writebuffered(c) && 
jffs2_sum_active())"?

Best regards,
Thomas