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656e9007ef
Code that passes a 32-bit constant into cmpxchg() produces a harmless
sparse warning because of the truncation in the branch that is not taken:
fs/erofs/zdata.c: note: in included file (through /home/arnd/arm-soc/arch/arm/include/asm/cmpxchg.h, /home/arnd/arm-soc/arch/arm/include/asm/atomic.h, /home/arnd/arm-soc/include/linux/atomic.h, ...):
include/asm-generic/cmpxchg-local.h:29:33: warning: cast truncates bits from constant value (5f0ecafe becomes fe)
include/asm-generic/cmpxchg-local.h:33:34: warning: cast truncates bits from constant value (5f0ecafe becomes cafe)
include/asm-generic/cmpxchg-local.h:29:33: warning: cast truncates bits from constant value (5f0ecafe becomes fe)
include/asm-generic/cmpxchg-local.h:30:42: warning: cast truncates bits from constant value (5f0edead becomes ad)
include/asm-generic/cmpxchg-local.h:33:34: warning: cast truncates bits from constant value (5f0ecafe becomes cafe)
include/asm-generic/cmpxchg-local.h:34:44: warning: cast truncates bits from constant value (5f0edead becomes dead)
This was reported as a regression to Matt's recent __generic_cmpxchg_local
patch, though this patch only added more warnings on top of the ones
that were already there.
Rewording the truncation to use an explicit bitmask instead of a cast
to a smaller type avoids the warning but otherwise leaves the code
unchanged.
I had another look at why the cast is even needed for atomic_cmpxchg(),
and as Matt describes the problem here is that atomic_t contains a
signed 'int', but cmpxchg() takes an 'unsigned long' argument, and
converting between the two leads to a 64-bit sign-extension of
negative 32-bit atomics.
I checked the other implementations of arch_cmpxchg() and did not find
any others that run into the same problem as __generic_cmpxchg_local(),
but it's easy to be on the safe side here and always convert the
signed int into an unsigned int when calling arch_cmpxchg(), as this
will work even when any of the arch_cmpxchg() implementations run
into the same problem.
Fixes: 6246541522 ("locking/atomic: cmpxchg: Make __generic_cmpxchg_local compare against zero-extended 'old' value")
Reviewed-by: Matt Evans <mev@rivosinc.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
68 lines
1.5 KiB
C
68 lines
1.5 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __ASM_GENERIC_CMPXCHG_LOCAL_H
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#define __ASM_GENERIC_CMPXCHG_LOCAL_H
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#include <linux/types.h>
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#include <linux/irqflags.h>
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extern unsigned long wrong_size_cmpxchg(volatile void *ptr)
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__noreturn;
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/*
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* Generic version of __cmpxchg_local (disables interrupts). Takes an unsigned
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* long parameter, supporting various types of architectures.
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*/
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static inline unsigned long __generic_cmpxchg_local(volatile void *ptr,
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unsigned long old, unsigned long new, int size)
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{
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unsigned long flags, prev;
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/*
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* Sanity checking, compile-time.
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*/
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if (size == 8 && sizeof(unsigned long) != 8)
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wrong_size_cmpxchg(ptr);
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raw_local_irq_save(flags);
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switch (size) {
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case 1: prev = *(u8 *)ptr;
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if (prev == (old & 0xffu))
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*(u8 *)ptr = (new & 0xffu);
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break;
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case 2: prev = *(u16 *)ptr;
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if (prev == (old & 0xffffu))
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*(u16 *)ptr = (new & 0xffffu);
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break;
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case 4: prev = *(u32 *)ptr;
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if (prev == (old & 0xffffffffffu))
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*(u32 *)ptr = (new & 0xffffffffu);
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break;
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case 8: prev = *(u64 *)ptr;
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if (prev == old)
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*(u64 *)ptr = (u64)new;
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break;
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default:
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wrong_size_cmpxchg(ptr);
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}
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raw_local_irq_restore(flags);
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return prev;
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}
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/*
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* Generic version of __cmpxchg64_local. Takes an u64 parameter.
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*/
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static inline u64 __generic_cmpxchg64_local(volatile void *ptr,
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u64 old, u64 new)
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{
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u64 prev;
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unsigned long flags;
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raw_local_irq_save(flags);
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prev = *(u64 *)ptr;
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if (prev == old)
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*(u64 *)ptr = new;
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raw_local_irq_restore(flags);
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return prev;
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}
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#endif
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