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

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Date:   Wed, 28 Jun 2017 16:54:45 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ingo Molnar <mingo@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.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, Jun 28, 2017 at 05:24:24PM +0200, Thomas Gleixner wrote:
> 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.

Sure, that makes sense to me.

> 
> 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

Given we're gonig to clean things up, we may as well avoid the backwards
include of <asm-generic/atomic_instrumented.h>, whcih was only there as
a bodge:

For the UP arches we do:
# echo '#include <asm-generic/atomic_up.h>' >arch/xxx/include/asm/atomic.h
# mv include/asm-generic/atomic.h include/asm-generic/atomic_up.h

Then we add a <linux/atomic_instrumented.h>:

#ifndef __LINUX_ATOMIC_INSTRUMENTED_H
#define __LINUX_ATOMIC INSTRUMENTED_H

#include <asm/atomic.h>

#if CONFIG_ATOMIC_INSTRUMENTED_H
<instrumentation>
#endif

#endif /* __LINUX_ATOMIC_ARCH_H */

... and make <linux/atomic.h> incldue that rather than <asm/atomic.h>.

That way the instrumentation's orthogonal to the UP-ness of the arch,
and we can fold any other instrumentation in there, or later move it
directly into <linux/atomic.h>

Thanks,
Mark.