Merge tag 'cmpxchg.2024.05.11a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull cmpxchg updates from Paul McKenney:
 "Provide one-byte and two-byte cmpxchg() support on sparc32, parisc,
  and csky

  This provides native one-byte and two-byte cmpxchg() support for
  sparc32 and parisc, courtesy of Al Viro. This support is provided by
  the same hashed-array-of-locks technique used for the other atomic
  operations provided for these two platforms.

  There is also emulated one-byte cmpxchg() support for csky using a new
  cmpxchg_emu_u8() function that uses a four-byte cmpxchg() to emulate
  the one-byte variant.

  Similar patches for emulation of one-byte cmpxchg() for arc, sh, and
  xtensa have not yet received maintainer acks, so they are slated for
  the v6.11 merge window"

* tag 'cmpxchg.2024.05.11a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  csky: Emulate one-byte cmpxchg
  lib: Add one-byte emulation function
  parisc: add u16 support to cmpxchg()
  parisc: add missing export of __cmpxchg_u8()
  parisc: unify implementations of __cmpxchg_u{8,32,64}
  parisc: __cmpxchg_u32(): lift conversion into the callers
  sparc32: add __cmpxchg_u{8,16}() and teach __cmpxchg() to handle those sizes
  sparc32: unify __cmpxchg_u{32,64}
  sparc32: make the first argument of __cmpxchg_u64() volatile u64 *
  sparc32: make __cmpxchg_u32() return u32

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);