riscv: implement xor_unlock_is_negative_byte

Inspired by the riscv clear_bit_unlock(), this will surely be
more efficient than the generic one defined in filemap.c.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Albert Ou <[e-mail]>
Cc: Alexander Gordeev <[e-mail]>
Cc: Andreas Dilger <[e-mail]>
Cc: Christian Borntraeger <[e-mail]>
Cc: Christophe Leroy <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Ivan Kokshaysky <[e-mail]>
Cc: Matt Turner <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: Nicholas Piggin <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Cc: Paul Walmsley <[e-mail]>
Cc: Richard Henderson <[e-mail]>
Cc: Sven Schnelle <[e-mail]>
Cc: "Theodore Ts'o" <[e-mail]>
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: Vasily Gorbik <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

arch/riscv/include/asm/bitops.h

···
 	clear_bit_unlock(nr, addr);
 }
 
+static inline bool xor_unlock_is_negative_byte(unsigned long mask,
+		volatile unsigned long *addr)
+{
+	unsigned long res;
+	__asm__ __volatile__ (
+		__AMO(xor) ".rl %0, %2, %1"
+		: "=r" (res), "+A" (*addr)
+		: "r" (__NOP(mask))
+		: "memory");
+	return (res & BIT(7)) != 0;
+}
+#define xor_unlock_is_negative_byte xor_unlock_is_negative_byte
+
 #undef __test_and_op_bit
 #undef __op_bit
 #undef __NOP