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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>
116 lines
2.4 KiB
C
116 lines
2.4 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* Generic UP xchg and cmpxchg using interrupt disablement. Does not
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* support SMP.
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*/
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#ifndef __ASM_GENERIC_CMPXCHG_H
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#define __ASM_GENERIC_CMPXCHG_H
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#ifdef CONFIG_SMP
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#error "Cannot use generic cmpxchg on SMP"
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#endif
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#include <linux/types.h>
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#include <linux/irqflags.h>
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/*
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* This function doesn't exist, so you'll get a linker error if
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* something tries to do an invalidly-sized xchg().
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*/
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extern void __generic_xchg_called_with_bad_pointer(void);
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static inline
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unsigned long __generic_xchg(unsigned long x, volatile void *ptr, int size)
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{
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unsigned long ret, flags;
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switch (size) {
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case 1:
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#ifdef __xchg_u8
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return __xchg_u8(x, ptr);
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#else
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local_irq_save(flags);
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ret = *(volatile u8 *)ptr;
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*(volatile u8 *)ptr = (x & 0xffu);
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local_irq_restore(flags);
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return ret;
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#endif /* __xchg_u8 */
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case 2:
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#ifdef __xchg_u16
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return __xchg_u16(x, ptr);
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#else
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local_irq_save(flags);
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ret = *(volatile u16 *)ptr;
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*(volatile u16 *)ptr = (x & 0xffffu);
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local_irq_restore(flags);
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return ret;
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#endif /* __xchg_u16 */
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case 4:
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#ifdef __xchg_u32
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return __xchg_u32(x, ptr);
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#else
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local_irq_save(flags);
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ret = *(volatile u32 *)ptr;
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*(volatile u32 *)ptr = (x & 0xffffffffu);
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local_irq_restore(flags);
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return ret;
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#endif /* __xchg_u32 */
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#ifdef CONFIG_64BIT
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case 8:
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#ifdef __xchg_u64
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return __xchg_u64(x, ptr);
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#else
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local_irq_save(flags);
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ret = *(volatile u64 *)ptr;
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*(volatile u64 *)ptr = x;
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local_irq_restore(flags);
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return ret;
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#endif /* __xchg_u64 */
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#endif /* CONFIG_64BIT */
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default:
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__generic_xchg_called_with_bad_pointer();
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return x;
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}
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}
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#define generic_xchg(ptr, x) ({ \
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((__typeof__(*(ptr))) \
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__generic_xchg((unsigned long)(x), (ptr), sizeof(*(ptr)))); \
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})
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/*
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* Atomic compare and exchange.
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*/
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#include <asm-generic/cmpxchg-local.h>
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#define generic_cmpxchg_local(ptr, o, n) ({ \
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((__typeof__(*(ptr)))__generic_cmpxchg_local((ptr), (unsigned long)(o), \
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(unsigned long)(n), sizeof(*(ptr)))); \
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})
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#define generic_cmpxchg64_local(ptr, o, n) \
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__generic_cmpxchg64_local((ptr), (o), (n))
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#ifndef arch_xchg
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#define arch_xchg generic_xchg
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#endif
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#ifndef arch_cmpxchg_local
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#define arch_cmpxchg_local generic_cmpxchg_local
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#endif
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#ifndef arch_cmpxchg64_local
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#define arch_cmpxchg64_local generic_cmpxchg64_local
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#endif
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#define arch_cmpxchg arch_cmpxchg_local
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#define arch_cmpxchg64 arch_cmpxchg64_local
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#endif /* __ASM_GENERIC_CMPXCHG_H */
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