linux-kernel - Re: [RESEND PATCH v2] devres: Really align data field to unsigned long long

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Date:   Mon, 9 Jul 2018 08:37:03 +0000
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     "geert@...ux-m68k.org" <geert@...ux-m68k.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "greg@...ah.com" <greg@...ah.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: [RESEND PATCH v2] devres: Really align data field to unsigned
 long long

Hi Geert,

On Mon, 2018-07-09 at 09:52 +0200, Geert Uytterhoeven wrote:
> Hi Alexey,
> 
> On Mon, Jul 9, 2018 at 9:22 AM Alexey Brodkin
> <Alexey.Brodkin@...opsys.com> wrote:
> > On Mon, 2018-07-09 at 09:07 +0200, Geert Uytterhoeven wrote:
> > > On Mon, Jul 9, 2018 at 7:49 AM Greg KH <greg@...ah.com> wrote:
> > > > On Mon, Jul 09, 2018 at 07:44:44AM +0300, Alexey Brodkin wrote:
> > > > > Depending on ABI "long long" type of a particular 32-bit CPU
> > > > > might be aligned by either word (32-bits) or double word (64-bits).
> > > 
> > > Or even 16-bit (on e.g. m68k).
> > 
> > Indeed, thanks for the note!
> > Will add this in my v3.
> 
> Note that in this particular case, the field will probably always be
> aligned to 4 bytes,
> as the struct will be allocated using *alloc().

Well I think maybe it worth mentioning only 32-bit and 64-bit alignments in
this particular case because as it was mentioned initially in the comment there're
3 pointers before "data" field and for 32-bit machines I guess we may safely
assume that a pointer size is 32-bit.

This leaves us only one problematic situation 32-bit instead of 64-bit alignment.

> > For ARC I'd like this fix to be back-ported starting
> > from v4.8 where we added support of "native" atomic64_t, see commit
> > ce6365270ecd (" ARCv2: Implement atomic64 based on LLOCKD/SCONDD instructions").
> > 
> > What about m68k, do you have any preference of earliest kernel version
> > where this fix might be useful?
> 
> Given m68k is 32-bit, it will access atomic64_t variables while
> holding a spinlock,
> so it should still be safe without this change.

Well ARC and ARMv7 are 32-bit machines as well still both have
a special instructions to read/write 64-bit data.

> Not to mention no one will try etnaviv on m68k ;-)

See it was just a trigger case but other GPUs that use or will use
DRM GPU scheduler are good candidates to it that problem and not to
mention some other users of atomic64_t.

But from you above comment I may deduce that m68k doesn't have
instructions for 64-bit data; in that case there's no reason to worry
at least about struct devres_node :)

-Alexey