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McKenney" To: Boqun Feng 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 Subject: Re: [PATCH v2 cmpxchg 09/13] lib: Add one-byte emulation function Message-ID: Reply-To: paulmck@kernel.org References: <20240501230130.1111603-9-paulmck@kernel.org> <9f0ff126-2806-488e-97cc-7258eff0c574@paulmck-laptop> <29f1d801-9fb4-4ecb-8d5e-cecb7d7a76e1@paulmck-laptop> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 > > > > > > > +#include > > > > > > > +#include > > > > > > > +#include > > > > > > > +#include > > > > > > > +#include > > > > > > > +#include > > > > > > > +#include > > > > > > > + > > > > > > > +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 > > > --- > > > 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 > > > + > > > +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 > > >