From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Alexey Brodkin <Alexey.Brodkin@synopsys.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
arcml <linux-snps-arc@lists.infradead.org>,
Linux-Arch <linux-arch@vger.kernel.org>,
Greg KH <gregkh@linuxfoundation.org>,
David Laight <David.Laight@aculab.com>,
Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <will.deacon@arm.com>, Greg KH <greg@kroah.com>,
stable <stable@vger.kernel.org>
Subject: Re: [PATCH v3] devres: Explicitly align datai[] to 64-bit
Date: Mon, 9 Jul 2018 16:10:23 +0200 [thread overview]
Message-ID: <CAMuHMdUS1QXuV8L1zadJ4XP=2g4pfoXgKByZ7Yg9PEbXOT_KVw@mail.gmail.com> (raw)
Message-ID: <20180709141023.LXemXzZ-859U1bEpE7S0WiKsHrfkWmwPOZxnAzAnCXs@z> (raw)
In-Reply-To: <20180709140438.ashvw4otyusj3nfa@lakrids.cambridge.arm.com>
On Mon, Jul 9, 2018 at 4:04 PM Mark Rutland <mark.rutland@arm.com> wrote:
> On Mon, Jul 09, 2018 at 03:54:09PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 09, 2018 at 04:45:50PM +0300, Alexey Brodkin wrote:
> > > diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> > > index f98a097e73f2..d65327cb83c9 100644
> > > --- a/drivers/base/devres.c
> > > +++ b/drivers/base/devres.c
> > > @@ -24,8 +24,12 @@ struct devres_node {
> > >
> > > struct devres {
> > > struct devres_node node;
> > > - /* -- 3 pointers */
> > > - unsigned long long data[]; /* guarantee ull alignment */
> > > + /*
> > > + * data[] must be 64 bit aligned even on 32 bit architectures
> > > + * because it might be accessed by instructions that require
> > > + * aligned memory arguments such as atomic64_t.
> > > + */
> > > + u8 __aligned(8) data[];
> > > };
> >
> > From a quick reading in Documentation/driver-model/devres.txt this
> > devres muck is supposed to be device memory, right?
>
> It's for associating resources (e.g. memory allocations) with a struct
> device.
>
> e.g. you do:
>
> devm_kmalloc(dev, size, GFP_KERNEL);
>
> ... and that allocates sizeof(struct devres) + size, putting some
> accounting data into that devres, and returning a pointer to the
> remaining size bytes.
>
> The data[] thing is a hack to ensure that the structure is padded to
> 64-bit alignment, in case you'd done:
>
> struct foo {
> atomic64_t counter;
> }
>
> struct foo *f = devm_kmalloc(dev, sizeof(*f), GFP_KERNEL);
So the big issue is that the minimum alignment of a buffer allocated with
devm_kmalloc() and friends is different (lower) than when allocated with
kmalloc().
On 32-bit, it's only aligned to 4 bytes. Ugh.
I wouldn't be surprised if some callers assume it to be cacheline-aligned...
Which means blind conversions to the devm_*() versions can be dangerous.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
next prev parent reply other threads:[~2018-07-09 14:10 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-09 13:45 [PATCH v3] devres: Explicitly align datai[] to 64-bit Alexey Brodkin
2018-07-09 13:45 ` Alexey Brodkin
2018-07-09 13:54 ` Peter Zijlstra
2018-07-09 13:54 ` Peter Zijlstra
2018-07-09 14:04 ` Mark Rutland
2018-07-09 14:04 ` Mark Rutland
2018-07-09 14:10 ` Geert Uytterhoeven [this message]
2018-07-09 14:10 ` Geert Uytterhoeven
2018-07-09 14:07 ` Alexey Brodkin
2018-07-09 14:07 ` Alexey Brodkin
2018-07-09 14:07 ` Peter Zijlstra
2018-07-09 14:07 ` Peter Zijlstra
2018-07-09 14:10 ` Peter Zijlstra
2018-07-09 14:10 ` Peter Zijlstra
2018-07-09 14:33 ` Alexey Brodkin
2018-07-09 14:33 ` Alexey Brodkin
2018-07-09 14:49 ` Peter Zijlstra
2018-07-09 14:49 ` Peter Zijlstra
2018-07-09 14:53 ` Alexey Brodkin
2018-07-09 14:53 ` Alexey Brodkin
2018-10-04 16:19 ` Alexey Brodkin
2018-10-04 16:19 ` Alexey Brodkin
2018-07-09 15:02 ` David Laight
2018-07-09 15:02 ` David Laight
2018-07-09 15:14 ` Peter Zijlstra
2018-07-09 15:14 ` Peter Zijlstra
2018-07-09 15:29 ` Mark Rutland
2018-07-09 15:29 ` Mark Rutland
2018-07-09 15:34 ` Peter Zijlstra
2018-07-09 15:34 ` Peter Zijlstra
2018-07-09 15:45 ` Peter Zijlstra
2018-07-09 15:45 ` Peter Zijlstra
2018-07-09 15:48 ` Mark Rutland
2018-07-09 15:48 ` Mark Rutland
2018-07-09 16:10 ` David Laight
2018-07-09 16:10 ` David Laight
2018-07-09 20:28 ` Vineet Gupta
2018-07-09 20:28 ` Vineet Gupta
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