linux-kernel - Re: [PATCH v2 cmpxchg 09/13] lib: Add one-byte emulation function

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Date: Mon, 13 May 2024 08:57:16 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	elver@...gle.com, akpm@...ux-foundation.org, tglx@...utronix.de,
	peterz@...radead.org, dianders@...omium.org, pmladek@...e.com,
	arnd@...db.de, torvalds@...ux-foundation.org, kernel-team@...a.com,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2 cmpxchg 09/13] lib: Add one-byte emulation function

On Mon, May 13, 2024 at 08:41:27AM -0700, Paul E. McKenney wrote:
[...]
> > > +#include <linux/types.h>
> > > +#include <linux/export.h>
> > > +#include <linux/instrumented.h>
> > > +#include <linux/atomic.h>
> > > +#include <linux/panic.h>
> > > +#include <linux/bug.h>
> > > +#include <asm-generic/rwonce.h>
> > > +#include <linux/cmpxchg-emu.h>
> > > +
> > > +union u8_32 {
> > > +	u8 b[4];
> > > +	u32 w;
> > > +};
> > > +
> > > +/* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */
> > > +uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
> > > +{
> > > +	u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
> > > +	int i = ((uintptr_t)p) & 0x3;
> > > +	union u8_32 old32;
> > > +	union u8_32 new32;
> > > +	u32 ret;
> > > +
> > > +	ret = READ_ONCE(*p32);
> > > +	do {
> > > +		old32.w = ret;
> > > +		if (old32.b[i] != old)
> > > +			return old32.b[i];
> > > +		new32.w = old32.w;
> > > +		new32.b[i] = new;
> > > +		instrument_atomic_read_write(p, 1);
> > > +		ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.
> > 
> > Just out of curiosity, why is this `data_race` needed? cmpxchg is atomic
> > so there should be no chance for a data race?
> 
> That is what I thought, too.  ;-)
> 
> The problem is that the cmpxchg() covers 32 bits, and so without that
> data_race(), KCSAN would complain about data races with perfectly
> legitimate concurrent accesses to the other three bytes.
> 
> The instrument_atomic_read_write(p, 1) beforehand tells KCSAN to complain
> about concurrent accesses, but only to that one byte.
> 

Oh, I see. For that purpose, maybe we can just use raw_cmpxchg() here,
i.e. a cmpxchg() without any instrument in it. Cc Mark in case I'm
missing something.

Regards,
Boqun

> 							Thanx, Paul
>