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

Paul E. McKenney
2024-05-14T15:02:55Z

On Tue, May 14, 2024 at 07:53:20AM -0700, Boqun Feng wrote:
> 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
> ;-)

I believe you, but could you nevertheless please test it? ;-)

Thanx, Paul

> 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
> > >