aboutsummaryrefslogtreecommitdiff
path: root/lib
diff options
context:
space:
mode:
authorPaul E. McKenney2024-03-17 14:44:38 -0700
committerPaul E. McKenney2024-04-09 22:06:00 -0700
commita88d970c8bb55dc20f319ce3e33c7ce0e9647480 (patch)
tree3018a3725478213510ae0620aac3c54ca6996149 /lib
parentd428032b3524e8fc75cf0bcc86b409df8ddf532b (diff)
lib: Add one-byte emulation function
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>
Diffstat (limited to 'lib')
-rw-r--r--lib/Makefile1
-rw-r--r--lib/cmpxchg-emu.c45
2 files changed, 46 insertions, 0 deletions
diff --git a/lib/Makefile b/lib/Makefile
index ffc6b2341b45..cc3d52fdb477 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -236,6 +236,7 @@ obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
lib-$(CONFIG_GENERIC_BUG) += bug.o
obj-$(CONFIG_HAVE_ARCH_TRACEHOOK) += syscall.o
+obj-$(CONFIG_ARCH_NEED_CMPXCHG_1_EMU) += cmpxchg-emu.o
obj-$(CONFIG_DYNAMIC_DEBUG_CORE) += dynamic_debug.o
#ensure exported functions have prototypes
diff --git a/lib/cmpxchg-emu.c b/lib/cmpxchg-emu.c
new file mode 100644
index 000000000000..27f6f97cb60d
--- /dev/null
+++ b/lib/cmpxchg-emu.c
@@ -0,0 +1,45 @@
+// 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);