linux-kernel - Re: [PATCH] locking/atomics: don't alias ___

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Date:   Wed, 28 Jun 2017 15:20:36 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ingo Molnar <mingo@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] locking/atomics: don't alias ____ptr

On Wed, 28 Jun 2017, Sebastian Andrzej Siewior wrote:
> On 2017-06-28 14:15:18 [+0300], Andrey Ryabinin wrote:
> > The main problem here is that arch_cmpxchg64_local() calls cmpxhg_local() instead of using arch_cmpxchg_local().
> > 
> > So, the patch bellow should fix the problem, also this will fix double instrumentation of cmpcxchg64[_local]().
> > But I haven't tested this patch yet.
> 
> tested, works. Next step?

Check all other implementations in every architecture whether there is a
similar problem .....

But this really want's a proper cleanup unless we want to waste the time
over and over again with the next hard to figure out macro expansion fail.

First of all, cmpxchg64[_local]() can be implemented as inlines right away.

For cmpxchg*(), the situation is slightly different, but the sizeof()
evaluation should be done at the top most level, even if we do it further
down in the low level arch/asm-generic implementation once more.

Something along the lines of:

static inline unsigned long cmpxchg_varsize(void *ptr, unsigned long old,
					    unsigned long new, int size)
{
	switch (size) {
	case 1:
	case 2:
	case 4:
		break;
	case 8:
		if (sizeof(unsigned long) == 8)
			break;
	default:
		BUILD_BUG_ON(1);
	}
	kasan_check(ptr, size);
	return arch_cmpxchg(ptr, old, new);
}

#define cmpxchg(ptr, o, n)						\
({									\
	((__typeof__(*(ptr)))cmpxchg_varsize((ptr), (unsigned long)(o), \
			     (unsigned long)(n), sizeof(*(ptr))));	\
})

That's the first step to cure the actual mess.

Ideally we get rid of that whole macro maze and convert everything to
proper inlines with actual cmpxchg8/16/32/64() variants, but that's going
to take some time. As an intermediate step we can at least propagate 'size'
to arch_cmpxchg(), which is not that much of an effort.

Thanks,

	tglx