lib/cmpxchg-emu.c

   1 // SPDX-License-Identifier: GPL-2.0+
   2 /*
   3  * Emulated 1-byte cmpxchg operation for architectures lacking direct
   4  * support for this size.  This is implemented in terms of 4-byte cmpxchg
   5  * operations.
   6  *
   7  * Copyright (C) 2024 Paul E. McKenney.
   8  */
   9
  10 #include <linux/types.h>
  11 #include <linux/export.h>
  12 #include <linux/instrumented.h>
  13 #include <linux/atomic.h>
  14 #include <linux/panic.h>
  15 #include <linux/bug.h>
  16 #include <asm-generic/rwonce.h>
  17 #include <linux/cmpxchg-emu.h>
  18
  19 union u8_32 {
  20         u8 b[4];
  21         u32 w;
  22 };
  23
  24 /* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */
  25 uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
  26 {
  27         u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
  28         int i = ((uintptr_t)p) & 0x3;
  29         union u8_32 old32;
  30         union u8_32 new32;
  31         u32 ret;
  32
  33         ret = READ_ONCE(*p32);
  34         do {
  35                 old32.w = ret;
  36                 if (old32.b[i] != old)
  37                         return old32.b[i];
  38                 new32.w = old32.w;
  39                 new32.b[i] = new;
  40                 instrument_atomic_read_write(p, 1);
  41                 ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.
  42         } while (ret != old32.w);
  43         return old;
  44 }
  45 EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);