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Date: Tue, 14 May 2024 07:53:20 -0700
From: Boqun Feng <boqun.feng@gmail.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
	elver@google.com, akpm@linux-foundation.org, tglx@linutronix.de,
	peterz@infradead.org, dianders@chromium.org, pmladek@suse.com,
	arnd@arndb.de, torvalds@linux-foundation.org, kernel-team@meta.com,
	Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH v2 cmpxchg 09/13] lib: Add one-byte emulation function
Message-ID: <ZkN64LAeOfHAXyUM@boqun-archlinux>
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On Tue, May 14, 2024 at 07:22:47AM -0700, Paul E. McKenney wrote:
> On Mon, May 13, 2024 at 02:19:37PM -0700, Boqun Feng wrote:
> > On Mon, May 13, 2024 at 08:57:16AM -0700, Boqun Feng wrote:
> > > 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.
> > > 
> > 
> > I just realized that the KCSAN instrumentation is already done in
> > cmpxchg() layer:
> > 
> > 	#define cmpxchg(ptr, ...) \
> > 	({ \
> > 		typeof(ptr) __ai_ptr = (ptr); \
> > 		kcsan_mb(); \
> > 		instrument_atomic_read_write(__ai_ptr, sizeof(*__ai_ptr)); \
> > 		raw_cmpxchg(__ai_ptr, __VA_ARGS__); \
> > 	})
> > 
> > and, this function is lower in the layer, so it shouldn't have the
> > instrumentation itself. How about the following (based on today's RCU
> > dev branch)?
> 
> The raw_cmpxchg() looks nicer than the added data_race()!
> 
> One question below, though.
> 
> 							Thanx, Paul
> 
> > Regards,
> > Boqun
> > 
> > -------------------------------------------->8
> > Subject: [PATCH] lib: cmpxchg-emu: Make cmpxchg_emu_u8() noinstr
> > 
> > Currently, cmpxchg_emu_u8() is called via cmpxchg() or raw_cmpxchg()
> > which already makes the instrumentation decision:
> > 
> > * cmpxchg() case:
> > 
> > 	cmpxchg():
> > 	  kcsan_mb();
> > 	  instrument_atomic_read_write(...);
> > 	  raw_cmpxchg():
> > 	    arch_cmpxchg():
> > 	      cmpxchg_emu_u8();
> > 
> > ... should have KCSAN instrumentation.
> > 
> > * raw_cmpxchg() case:
> > 
> > 	raw_cmpxchg():
> > 	  arch_cmpxchg():
> > 	    cmpxchg_emu_u8();
> > 
> > ... shouldn't have KCSAN instrumentation.
> > 
> > Therefore it's redundant to put KCSAN instrumentation in
> > cmpxchg_emu_u8() (along with the data_race() to get away the
> > instrumentation).
> > 
> > So make cmpxchg_emu_u8() a noinstr function, and remove the KCSAN
> > instrumentation inside it.
> > 
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
> >  include/linux/cmpxchg-emu.h |  4 +++-
> >  lib/cmpxchg-emu.c           | 14 ++++++++++----
> >  2 files changed, 13 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/cmpxchg-emu.h b/include/linux/cmpxchg-emu.h
> > index 998deec67740..c4c85f41d9f4 100644
> > --- a/include/linux/cmpxchg-emu.h
> > +++ b/include/linux/cmpxchg-emu.h
> > @@ -10,6 +10,8 @@
> >  #ifndef __LINUX_CMPXCHG_EMU_H
> >  #define __LINUX_CMPXCHG_EMU_H
> >  
> > -uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new);
> > +#include <linux/compiler.h>
> > +
> > +noinstr uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new);
> >  
> >  #endif /* __LINUX_CMPXCHG_EMU_H */
> > diff --git a/lib/cmpxchg-emu.c b/lib/cmpxchg-emu.c
> > index 27f6f97cb60d..788c22cd4462 100644
> > --- a/lib/cmpxchg-emu.c
> > +++ b/lib/cmpxchg-emu.c
> > @@ -21,8 +21,13 @@ union u8_32 {
> >  	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)
> > +/*
> > + * Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg.
> > + *
> > + * This function is marked as 'noinstr' as the instrumentation should be done at
> > + * outer layer.
> > + */
> > +noinstr 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;
> > @@ -37,8 +42,9 @@ uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
> >  			return old32.b[i];
> >  		new32.w = old32.w;
> >  		new32.b[i] = new;
> > -		instrument_atomic_read_write(p, 1);
> 
> Don't we need to keep that instrument_atomic_read_write() in order
> to allow KCSAN to detect data races with plain C-language reads?
> Or is that being handled some other way?
> 

I think that's already covered by the current code, cmpxchg_emu_u8() is
called from cmpxchg() macro, which has a:

	instrument_atomic_read_write(p, sizeof(*p));

in it, and in the cmpxchg((u8*), ..) case, 'sizeof(*p)' is obviously 1
;-)

Regards,
Boqun

> > -		ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.
> > +
> > +		// raw_cmpxchg() is used here to avoid instrumentation.
> > +		ret = raw_cmpxchg(p32, old32.w, new32.w); // Overridden above.
> >  	} while (ret != old32.w);
> >  	return old;
> >  }
> > -- 
> > 2.44.0
> >