linux-kernel - Re: [PATCH 00/22] Add and use pci_zalloc

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Date:	Tue, 24 Jun 2014 14:11:22 +0200 (CEST)
From:	Julia Lawall <julia.lawall@...6.fr>
To:	Joe Perches <joe@...ches.com>
cc:	Julian Calaby <julian.calaby@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/22] Add and use pci_zalloc_consistent



On Tue, 24 Jun 2014, Joe Perches wrote:

> On Tue, 2014-06-24 at 07:24 +0200, Julia Lawall wrote
> > On Mon, 23 Jun 2014, Joe Perches wrote:
> > > > > $ cat ./scripts/coccinelle/api/alloc/pci_zalloc_consistent.cocci
> > > > > ///
> > > > > /// Use pci_zalloc_consistent rather than
> > > > > ///     pci_alloc_consistent followed by memset with 0
> > > > > ///
> > > > > /// This considers some simple cases that are common and easy to validate
> > > > > /// Note in particular that there are no ...s in the rule, so all of the
> > > > > /// matched code has to be contiguous
> > > > > ///
> > > > > /// Blatantly cribbed from: scripts/coccinelle/api/alloc/kzalloc-simple.cocci
> > > > >
> > > > > @@
> > > > > type T, T2;
> > > > > expression x;
> > > > > expression E1,E2,E3;
> > > > > statement S;
> > > > > @@
> > > > >
> > > > > - x = (T)pci_alloc_consistent(E1,E2,E3);
> > > > > + x = pci_zalloc_consistent(E1,E2,E3);
> > > > >   if ((x==NULL) || ...) S
> > > > > - memset((T2)x,0,E2);
> > > >
> > > > I don't know much about SmPL, but wouldn't having that if statement
> > > > there reduce your matches?
> > >
> > > No, not really.
> > >
> > > Almost none of the pci_alloc_consistent calls
> > > do not have a test for failure immediately after
> > > them.
>
> > Do not or do?
>
> Sorry, English double negative.
>
> As far as I know, almost every instance of pci_alloc_consistent
> is followed by an if.
>
> Exceptions exist in:
>
> drivers/isdn/hardware/eicon/divasmain.c
> drivers/staging/slicoss/slicoss.c
> drivers/tty/synclink_gt.c
>
> There might be others, but I didn't look too hard.
>
> > The advantage of the if is that you are sure that nothing strange happens
> > to x between alloc and memset.  But a problem can be that sometimes people
> > allocate two things, and then do error checking for both of them.  Then
> > you rule would not match.  Or the set the return variable to an error code
> > before doing the check rather than in the if branch.
> >
> > You could put the following between the malloc and the memset in stead of
> > the if.
> >
> > ... when != ( f(...,x,...) | <+...x...+> = E3 )
> >     when != ( while(...) S | for(...;...;...) S )
> >
> > This has given reasonable results for kmalloc and memset.
>
> Thanks for that.
>
> It might be nice to add that to the kzalloc example
> in scripts/coccinelle

I will do that.  Thanks.

julia
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