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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-488684863d9sm5350009f8f.12.2026.09.23.02.30.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 23 Sep 2026 02:30:28 -0700 (PDT) Date: Wed, 23 Sep 2026 10:30:27 +0100 From: David Laight To: Bradley Morgan Cc: Andrew Morton , Vineet Gupta , Guo Ren , Yoshinori Sato , Rich Felker , Chris Zankel , Max Filippov , Arnd Bergmann , "Paul E . McKenney" , John Paul Adrian Glaubitz , linux-snps-arc@lists.infradead.org, linux-csky@vger.kernel.org, linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/5] ARC: Emulate two-byte cmpxchg Message-ID: <20260923103027.22bd9ef4@pumpkin> In-Reply-To: <20260922173354.14404-3-brads@mainlining.org> References: <20260922173354.14404-1-brads@mainlining.org> <20260922173354.14404-3-brads@mainlining.org> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-sh@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 22 Sep 2026 17:33:51 +0000 Bradley Morgan wrote: > ARC has no two-byte atomic compare and swap, so the arch_cmpxchg_relaxed() > macro switch lets case 2 fall through to BUILD_BUG() via default. Route case > 2 through the new cmpxchg_emu_u16(), which takes the old and new > values as unsigned long and narrows them itself, so the (uintptr_t) > casts on _o_ and _n_ are no longer needed and the case 2 call passes > them straight. > > The switch also now tests sizeof(*(_p_)) instead of sizeof((_p_)), > which switched on the pointer and made the size 1 and size 2 cases > dead code, routing every sub-word cmpxchg() through the 32-bit > llock/scond pair and comparing whole words against sub-word values, > so the compare almost never succeeded. Hmmm... that looks like it warrants a 'Fixes' tag. Clearly no one uses 'arc'. David > The old and new values are > now declared as unsigned long through (unsigned long)(0 ? *(_p_) : > (old)), the idiom David Laight suggested, which type checks the > arguments against the pointee, so cmpxchg(&p, 4, 5) no longer > compiles silently. > > Signed-off-by: Bradley Morgan > --- > arch/arc/include/asm/cmpxchg.h | 19 +++++++++++-------- > 1 file changed, 11 insertions(+), 8 deletions(-) > > diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h > index 76f43db0890f..4596edfefa95 100644 > --- a/arch/arc/include/asm/cmpxchg.h > +++ b/arch/arc/include/asm/cmpxchg.h > @@ -42,16 +42,19 @@ > #define arch_cmpxchg_relaxed(ptr, old, new) \ > ({ \ > __typeof__(ptr) _p_ = (ptr); \ > - __typeof__(*(ptr)) _o_ = (old); \ > - __typeof__(*(ptr)) _n_ = (new); \ > + unsigned long _old_ = (unsigned long)(0 ? *(_p_) : (old)); \ > + unsigned long _new_ = (unsigned long)(0 ? *(_p_) : (new)); \ > __typeof__(*(ptr)) _prev_; \ > \ > - switch(sizeof((_p_))) { \ > + switch (sizeof(*(_p_))) { \ > case 1: \ > - _prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *__force)_p_, (uintptr_t)_o_, (uintptr_t)_n_); \ > + _prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *__force)_p_, _old_, _new_); \ > + break; \ > + case 2: \ > + _prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *__force)_p_, _old_, _new_); \ > break; \ > case 4: \ > - _prev_ = __cmpxchg(_p_, _o_, _n_); \ > + _prev_ = (__typeof__(*(ptr)))__cmpxchg(_p_, _old_, _new_); \ > break; \ > default: \ > BUILD_BUG(); \ > @@ -64,8 +67,8 @@ > #define arch_cmpxchg(ptr, old, new) \ > ({ \ > volatile __typeof__(ptr) _p_ = (ptr); \ > - __typeof__(*(ptr)) _o_ = (old); \ > - __typeof__(*(ptr)) _n_ = (new); \ > + unsigned long _old_ = (unsigned long)(0 ? *(_p_) : (old)); \ > + unsigned long _new_ = (unsigned long)(0 ? *(_p_) : (new)); \ > __typeof__(*(ptr)) _prev_; \ > unsigned long __flags; \ > \ > @@ -102,7 +105,7 @@ > __typeof__(ptr) _p_ = (ptr); \ > __typeof__(*(ptr)) _val_ = (val); \ > \ > - switch(sizeof(*(_p_))) { \ > + switch (sizeof(*(_p_))) { \ > case 4: \ > _val_ = __arch_xchg(_p_, _val_); \ > break; \