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Date: Wed, 28 Jun 2017 17:24:24 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Mark Rutland <mark.rutland@arm.com>
cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] locking/atomics: don't alias ____ptr
In-Reply-To: <20170628141420.GK5981@leverpostej>
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On Wed, 28 Jun 2017, Mark Rutland wrote:
> On Wed, Jun 28, 2017 at 03:54:42PM +0200, Thomas Gleixner wrote:
> > > 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);
> > > }
> 
> This'll need to re-cast things before the call to arch_cmpxchg(), and we
> can move the check above the switch, as in [2].

Sure, but I rather see that changed to:

1) Create arch_cmpxchg8/16/32/64() inlines first

2) Add that varsize wrapper:

static inline unsigned long cmpxchg_varsize(void *ptr, unsigned long old,
                                            unsigned long new, int size)
{
        switch (size) {
        case 1:
                kasan_check_write(ptr, size);
                return arch_cmpxchg8((u8 *)ptr, (u8) old, (u8)new);
        case 2:
                kasan_check_write(ptr, size);
                return arch_cmpxchg16((u16 *)ptr, (u16) old, (u16)new);
        case 4:
                kasan_check_write(ptr, size);
                return arch_cmpxchg32((u32 *)ptr, (u32) old, (u32)new);
        case 8:
                if (sizeof(unsigned long) == 8) {
                        kasan_check_write(ptr, size);
                        return arch_cmpxchg64((u64 *)ptr, (u64) old, (u64)new);
                }
        default:
                BUILD_BUG();
        }
}

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

Which allows us to create:

static inline u8 cmpxchg8(u8 *ptr, u8 old, u8 new)
{
	kasan_check_write(ptr, sizeof(old));
	return arch_cmpxchg8(ptr, old, new);
}

and friends as well and later migrate the existing users away from that
untyped macro mess.

And instead of adding

    #include <asm/atomic-instrumented.h>

to the architecture code, we rather do

# mv arch/xxx/include/asm/atomic.h mv arch/xxx/include/asm/arch_atomic.h
# echo '#include <asm-generic/atomic.h>' >arch/xxx/include/asm/atomic.h

# mv include/asm-generic/atomic.h include/asm-generic/atomic_up.h

and create a new include/asm-generic/atomic.h

#ifndef __ASM_GENERIC_ATOMIC_H
#define __ASM_GENERIC_ATOMIC_H

#ifdef CONFIG_ATOMIC_INSTRUMENTED_H
#include <asm-generic/atomic_instrumented.h>
#else
#include <asm-generic/atomic_up.h>
#endif

#endif

Thanks,

	tglx