On Thu, 2012-03-08 at 08:13 +0100, Ingo Molnar wrote:
> * Alex Shi <alex.shi@xxxxxxxxx> wrote:
>
> > On Wed, 2012-03-07 at 14:39 +0100, Ingo Molnar wrote:
> > > * Alex Shi <alex.shi@xxxxxxxxx> wrote:
> > >
> > > > > I think the check should be (__alignof__(lock) <
> > > > > __alignof__(rwlock_t)), otherwise it will still pass when
> > > > > you have structure with attribute((packed,aligned(2)))
> > > >
> > > > reasonable!
> > > >
> > > > >> 1, it is alignof bug for default gcc on my fc15 and Ubuntu 11.10 etc?
> > > > >>
> > > > >> struct sub {
> > > > >> int raw_lock;
> > > > >> char a;
> > > > >> };
> > > > >> struct foo {
> > > > >> struct sub z;
> > > > >> int slk;
> > > > >> char y;
> > > > >> }__attribute__((packed));
> > > > >>
> > > > >> struct foo f1;
> > > > >>
> > > > >> __alignof__(f1.z.raw_lock) is 4, but its address actually can align on
> > > > >> one byte.
> > > > >
> > > > > That looks like correct behavior, because the alignment of
> > > > > raw_lock inside of struct sub is still 4. But it does mean
> > > > > that there can be cases where the compile-time check is not
> > > > > sufficient, so we might want the run-time check as well, at
> > > > > least under some config option.
> > > >
> > > > what's your opinion of this, Ingo?
> > >
> > > Dunno. How many real bugs have you found via this patch?
> >
> > None. Guess stupid code was shot in lkml reviewing. But if the
> > patch in, it is helpful to block stupid code in developing.
>
> The question is, if in the last 10 years not a single such case
> made it through to today's 15 million lines of kernel code, why
> should we add the check now?
>
> If it was a simple build time check then maybe, but judging by
> the discussion it does not seem so simple, does it?
Oh, It is may better to have, but I don't mind it was slided. Since even
alignof works as our expectation, it also can't cover all problems.
Currently, we heavily depend gcc's behavior.
Anyway, thanks for review!
>
> Thanks,
>
> Ingo
