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// SPDX-License-Identifier: GPL-2.0+
/*
* Emulated 1-byte cmpxchg operation for architectures lacking direct
* support for this size. This is implemented in terms of 4-byte cmpxchg
* operations.
*
* Copyright (C) 2024 Paul E. McKenney.
*/
#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.
} while (ret != old32.w);
return old;
}
EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);
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