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Architectures are required to provide four-byte cmpxchg() and 64-bit architectures are additionally required to provide eight-byte cmpxchg(). However, there are cases where one-byte cmpxchg() would be extremely useful. Therefore, provide cmpxchg_emu_u8() that emulates one-byte cmpxchg() in terms of four-byte cmpxchg(). Note that this emulations is fully ordered, and can (for example) cause one-byte cmpxchg_relaxed() to incur the overhead of full ordering. If this causes problems for a given architecture, that architecture is free to provide its own lighter-weight primitives. [ paulmck: Apply Marco Elver feedback. ] [ paulmck: Apply kernel test robot feedback. ] [ paulmck: Drop two-byte support per Arnd Bergmann feedback. ] Link: https://lore.kernel.org/all/0733eb10-5e7a-4450-9b8a-527b97c842ff@paulmck-laptop/ Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Acked-by: Marco Elver <elver@google.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org> Cc: Douglas Anderson <dianders@chromium.org> Cc: Petr Mladek <pmladek@suse.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: <linux-arch@vger.kernel.org>
45 lines
1.1 KiB
C
45 lines
1.1 KiB
C
// SPDX-License-Identifier: GPL-2.0+
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/*
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* Emulated 1-byte cmpxchg operation for architectures lacking direct
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* support for this size. This is implemented in terms of 4-byte cmpxchg
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* operations.
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*
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* Copyright (C) 2024 Paul E. McKenney.
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*/
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#include <linux/types.h>
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#include <linux/export.h>
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#include <linux/instrumented.h>
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#include <linux/atomic.h>
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#include <linux/panic.h>
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#include <linux/bug.h>
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#include <asm-generic/rwonce.h>
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#include <linux/cmpxchg-emu.h>
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union u8_32 {
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u8 b[4];
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u32 w;
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};
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/* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */
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uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
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{
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u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
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int i = ((uintptr_t)p) & 0x3;
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union u8_32 old32;
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union u8_32 new32;
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u32 ret;
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ret = READ_ONCE(*p32);
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do {
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old32.w = ret;
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if (old32.b[i] != old)
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return old32.b[i];
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new32.w = old32.w;
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new32.b[i] = new;
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instrument_atomic_read_write(p, 1);
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ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.
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} while (ret != old32.w);
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return old;
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}
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EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);
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