futex: Ensure futex_atomic_cmpxchg_inatomic() is present

The boot-time detection of futex_atomic_cmpxchg_inatomic() has a bug on
some 32-bit arm builds, and Thomas Gleixner suggested that setting
CONFIG_HAVE_FUTEX_CMPXCHG would avoid the problem, as it is always present
anyway.

Looking into which other architectures could do the same showed that almost
all architectures have it, the exceptions being:

 - some old 32-bit MIPS uniprocessor cores without ll/sc
 - one xtensa variant with no SMP
 - 32-bit SPARC when built for SMP

Fix MIPS And Xtensa by rearranging the generic code to let it be used
as a fallback.

For SPARC, the SMP definition just ends up turning off futex anyway, so
this can be done at Kconfig time instead. Note that sparc32 glibc requires
the CASA instruction for its mutexes anyway, which is only available when
running on SPARCv9 or LEON CPUs, but needs to be implemented in the sparc32
kernel for those.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Rich Felker <dalias@libc.org>
Link: https://lore.kernel.org/r/20211026100432.1730393-1-arnd@kernel.org

arch/mips/include/asm/futex.h

@@ -19,7 +19,11 @@
 #include <asm/sync.h>
 #include <asm/war.h>
 
-#define __futex_atomic_op(insn, ret, oldval, uaddr, oparg)		\
+#define arch_futex_atomic_op_inuser arch_futex_atomic_op_inuser
+#define futex_atomic_cmpxchg_inatomic futex_atomic_cmpxchg_inatomic
+#include <asm-generic/futex.h>
+
+#define __futex_atomic_op(op, insn, ret, oldval, uaddr, oparg)		\
 {									\
 	if (cpu_has_llsc && IS_ENABLED(CONFIG_WAR_R10000_LLSC)) {	\
 		__asm__ __volatile__(					\
@@ -80,9 +84,11 @@
 		: "0" (0), GCC_OFF_SMALL_ASM() (*uaddr), "Jr" (oparg),	\
 		  "i" (-EFAULT)						\
 		: "memory");						\
-	} else								\
+	} else {							\
-		ret = -ENOSYS;						\
+		/* fallback for non-SMP */				\
-}
+		ret = arch_futex_atomic_op_inuser_local(op, oparg, oval,\
+							uaddr);	\
+	}
 
 static inline int
 arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
@@ -94,23 +100,23 @@ arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr)
 
 	switch (op) {
 	case FUTEX_OP_SET:
-		__futex_atomic_op("move $1, %z5", ret, oldval, uaddr, oparg);
+		__futex_atomic_op(op, "move $1, %z5", ret, oldval, uaddr, oparg);
 		break;
 
 	case FUTEX_OP_ADD:
-		__futex_atomic_op("addu $1, %1, %z5",
+		__futex_atomic_op(op, "addu $1, %1, %z5",
 				  ret, oldval, uaddr, oparg);
 		break;
 	case FUTEX_OP_OR:
-		__futex_atomic_op("or	$1, %1, %z5",
+		__futex_atomic_op(op, "or	$1, %1, %z5",
 				  ret, oldval, uaddr, oparg);
 		break;
 	case FUTEX_OP_ANDN:
-		__futex_atomic_op("and	$1, %1, %z5",
+		__futex_atomic_op(op, "and	$1, %1, %z5",
 				  ret, oldval, uaddr, ~oparg);
 		break;
 	case FUTEX_OP_XOR:
-		__futex_atomic_op("xor	$1, %1, %z5",
+		__futex_atomic_op(op, "xor	$1, %1, %z5",
 				  ret, oldval, uaddr, oparg);
 		break;
 	default:
@@ -193,8 +199,9 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 		: GCC_OFF_SMALL_ASM() (*uaddr), "Jr" (oldval), "Jr" (newval),
 		  "i" (-EFAULT)
 		: "memory");
-	} else
+	} else {
-		return -ENOSYS;
+		return futex_atomic_cmpxchg_inatomic_local(uval, uaddr, oldval, newval);
+	}
 
 	*uval = val;
 	return ret;

arch/xtensa/include/asm/futex.h

@@ -16,6 +16,10 @@
 #include <linux/uaccess.h>
 #include <linux/errno.h>
 
+#define arch_futex_atomic_op_inuser arch_futex_atomic_op_inuser
+#define futex_atomic_cmpxchg_inatomic futex_atomic_cmpxchg_inatomic
+#include <asm-generic/futex.h>
+
 #if XCHAL_HAVE_EXCLUSIVE
 #define __futex_atomic_op(insn, ret, old, uaddr, arg)	\
 	__asm__ __volatile(				\
@@ -105,7 +109,7 @@ static inline int arch_futex_atomic_op_inuser(int op, int oparg, int *oval,
 
 	return ret;
 #else
-	return -ENOSYS;
+	return arch_futex_atomic_op_inuser_local(op, oparg, oval, uaddr);
 #endif
 }
 
@@ -156,7 +160,7 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 
 	return ret;
 #else
-	return -ENOSYS;
+	return futex_atomic_cmpxchg_inatomic_local(uval, uaddr, oldval, newval);
 #endif
 }
 

include/asm-generic/futex.h

@@ -6,15 +6,22 @@
 #include <linux/uaccess.h>
 #include <asm/errno.h>
 
+#ifndef futex_atomic_cmpxchg_inatomic
 #ifndef CONFIG_SMP
 /*
  * The following implementation only for uniprocessor machines.
  * It relies on preempt_disable() ensuring mutual exclusion.
  *
  */
+#define futex_atomic_cmpxchg_inatomic(uval, uaddr, oldval, newval) \
+	futex_atomic_cmpxchg_inatomic_local_generic(uval, uaddr, oldval, newval)
+#define arch_futex_atomic_op_inuser(op, oparg, oval, uaddr) \
+	arch_futex_atomic_op_inuser_local_generic(op, oparg, oval, uaddr)
+#endif /* CONFIG_SMP */
+#endif
 
 /**
- * arch_futex_atomic_op_inuser() - Atomic arithmetic operation with constant
+ * arch_futex_atomic_op_inuser_local() - Atomic arithmetic operation with constant
  *			  argument and comparison of the previous
  *			  futex value with another constant.
  *
@@ -28,7 +35,7 @@
  * -ENOSYS - Operation not supported
  */
 static inline int
-arch_futex_atomic_op_inuser(int op, u32 oparg, int *oval, u32 __user *uaddr)
+futex_atomic_op_inuser_local(int op, u32 oparg, int *oval, u32 __user *uaddr)
 {
 	int oldval, ret;
 	u32 tmp;
@@ -75,7 +82,7 @@ out_pagefault_enable:
 }
 
 /**
- * futex_atomic_cmpxchg_inatomic() - Compare and exchange the content of the
+ * futex_atomic_cmpxchg_inatomic_local() - Compare and exchange the content of the
  *				uaddr with newval if the current value is
  *				oldval.
  * @uval:	pointer to store content of @uaddr
@@ -87,10 +94,9 @@ out_pagefault_enable:
  * 0 - On success
  * -EFAULT - User access resulted in a page fault
  * -EAGAIN - Atomic operation was unable to complete due to contention
- * -ENOSYS - Function not implemented (only if !HAVE_FUTEX_CMPXCHG)
  */
 static inline int
-futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
+futex_atomic_cmpxchg_inatomic_local(u32 *uval, u32 __user *uaddr,
 			      u32 oldval, u32 newval)
 {
 	u32 val;
@@ -112,19 +118,4 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 	return 0;
 }
 
-#else
-static inline int
-arch_futex_atomic_op_inuser(int op, u32 oparg, int *oval, u32 __user *uaddr)
-{
-	return -ENOSYS;
-}
-
-static inline int
-futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
-			      u32 oldval, u32 newval)
-{
-	return -ENOSYS;
-}
-
-#endif /* CONFIG_SMP */
 #endif

init/Kconfig

@@ -1579,6 +1579,7 @@ config BASE_FULL
 
 config FUTEX
 	bool "Enable futex support" if EXPERT
+	depends on !(SPARC32 && SMP)
 	default y
 	imply RT_MUTEXES
 	help