[PATCH 2/2] futex: remove futex_cmpxchg detection

[PATCH 2/2] futex: remove futex_cmpxchg detection

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

Now that all architectures have a working futex implementation
in any configuration, remove the runtime detection code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 arch/arc/Kconfig              |  1 -
 arch/arm/Kconfig              |  1 -
 arch/arm64/Kconfig            |  1 -
 arch/csky/Kconfig             |  1 -
 arch/m68k/Kconfig             |  1 -
 arch/riscv/Kconfig            |  1 -
 arch/s390/Kconfig             |  1 -
 arch/sh/Kconfig               |  1 -
 arch/um/Kconfig               |  1 -
 arch/um/kernel/skas/uaccess.c |  1 -
 arch/xtensa/Kconfig           |  1 -
 init/Kconfig                  |  8 --------
 kernel/futex/core.c           | 35 -----------------------------------
 kernel/futex/futex.h          |  6 ------
 kernel/futex/syscalls.c       | 22 ----------------------
 15 files changed, 82 deletions(-)

diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index 248389278e8f..f9413041686f 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -31,7 +31,6 @@ config ARC
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARC_MMU_V4
 	select HAVE_DEBUG_STACKOVERFLOW
 	select HAVE_DEBUG_KMEMLEAK
-	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select HAVE_IOREMAP_PROT
 	select HAVE_KERNEL_GZIP
 	select HAVE_KERNEL_LZMA
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index bb5d2c45477b..6448d311635d 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -93,7 +93,6 @@ config ARM
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
 	select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
 	select HAVE_FUNCTION_TRACER if !XIP_KERNEL && !(THUMB2_KERNEL && CC_IS_CLANG)
-	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select HAVE_GCC_PLUGINS
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
 	select HAVE_IRQ_TIME_ACCOUNTING
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 0efc501f77aa..6c3c2ff5cef8 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -193,7 +193,6 @@ config ARM64
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_FUNCTION_ARG_ACCESS_API
-	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select MMU_GATHER_RCU_TABLE_FREE
 	select HAVE_RSEQ
 	select HAVE_STACKPROTECTOR
diff --git a/arch/csky/Kconfig b/arch/csky/Kconfig
index 823d3d5a9e11..efd7c5feac8b 100644
--- a/arch/csky/Kconfig
+++ b/arch/csky/Kconfig
@@ -53,7 +53,6 @@ config CSKY
 	select HAVE_FUNCTION_TRACER
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_ERROR_INJECTION
-	select HAVE_FUTEX_CMPXCHG if FUTEX && SMP
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_KERNEL_GZIP
 	select HAVE_KERNEL_LZO
diff --git a/arch/m68k/Kconfig b/arch/m68k/Kconfig
index 0b50da08a9c5..15a793c5b2dc 100644
--- a/arch/m68k/Kconfig
+++ b/arch/m68k/Kconfig
@@ -20,7 +20,6 @@ config M68K
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_DEBUG_BUGVERBOSE
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if !CPU_HAS_NO_UNALIGNED
-	select HAVE_FUTEX_CMPXCHG if MMU && FUTEX
 	select HAVE_MOD_ARCH_SPECIFIC
 	select HAVE_UID16
 	select MMU_GATHER_NO_RANGE if MMU
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 77a088d0a7e9..037fea9fac14 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -84,7 +84,6 @@ config RISCV
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_EBPF_JIT if MMU
 	select HAVE_FUNCTION_ERROR_INJECTION
-	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select HAVE_GCC_PLUGINS
 	select HAVE_GENERIC_VDSO if MMU && 64BIT
 	select HAVE_IRQ_TIME_ACCOUNTING
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index f615c3f65f5a..1c9ecf619e04 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -164,7 +164,6 @@ config S390
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
-	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select HAVE_GCC_PLUGINS
 	select HAVE_GENERIC_VDSO
 	select HAVE_IOREMAP_PROT if PCI
diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
index 6904f4bdbf00..93195d3368c0 100644
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -34,7 +34,6 @@ config SUPERH
 	select HAVE_FAST_GUP if MMU
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
-	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_HW_BREAKPOINT
 	select HAVE_IOREMAP_PROT if MMU && !X2TLB
diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index c18b45f75d41..c906250d4970 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -14,7 +14,6 @@ config UML
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_UID16
-	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DEBUG_BUGVERBOSE
 	select NO_DMA if !UML_DMA_EMULATION
diff --git a/arch/um/kernel/skas/uaccess.c b/arch/um/kernel/skas/uaccess.c
index a509be911026..9e37a7c05990 100644
--- a/arch/um/kernel/skas/uaccess.c
+++ b/arch/um/kernel/skas/uaccess.c
@@ -348,7 +348,6 @@ EXPORT_SYMBOL(arch_futex_atomic_op_inuser);
  * 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)
  */
 
 int futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
index 0e56bad058fa..8ac599aa6d99 100644
--- a/arch/xtensa/Kconfig
+++ b/arch/xtensa/Kconfig
@@ -31,7 +31,6 @@ config XTENSA
 	select HAVE_DMA_CONTIGUOUS
 	select HAVE_EXIT_THREAD
 	select HAVE_FUNCTION_TRACER
-	select HAVE_FUTEX_CMPXCHG if !MMU && FUTEX
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS
 	select HAVE_IRQ_TIME_ACCOUNTING
 	select HAVE_PCI
diff --git a/init/Kconfig b/init/Kconfig
index c0f55ea5a71f..538688598f2f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1597,14 +1597,6 @@ config FUTEX_PI
 	depends on FUTEX && RT_MUTEXES
 	default y
 
-config HAVE_FUTEX_CMPXCHG
-	bool
-	depends on FUTEX
-	help
-	  Architectures should select this if futex_atomic_cmpxchg_inatomic()
-	  is implemented and always working. This removes a couple of runtime
-	  checks.
-
 config EPOLL
 	bool "Enable eventpoll support" if EXPERT
 	default y
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 25d8a88b32e5..926c2bb752bc 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -41,11 +41,6 @@
 #include "futex.h"
 #include "../locking/rtmutex_common.h"
 
-#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
-int  __read_mostly futex_cmpxchg_enabled;
-#endif
-
-
 /*
  * The base of the bucket array and its size are always used together
  * (after initialization only in futex_hash()), so ensure that they
@@ -776,9 +771,6 @@ static void exit_robust_list(struct task_struct *curr)
 	unsigned long futex_offset;
 	int rc;
 
-	if (!futex_cmpxchg_enabled)
-		return;
-
 	/*
 	 * Fetch the list head (which was registered earlier, via
 	 * sys_set_robust_list()):
@@ -874,9 +866,6 @@ static void compat_exit_robust_list(struct task_struct *curr)
 	compat_long_t futex_offset;
 	int rc;
 
-	if (!futex_cmpxchg_enabled)
-		return;
-
 	/*
 	 * Fetch the list head (which was registered earlier, via
 	 * sys_set_robust_list()):
@@ -950,8 +939,6 @@ static void exit_pi_state_list(struct task_struct *curr)
 	struct futex_hash_bucket *hb;
 	union futex_key key = FUTEX_KEY_INIT;
 
-	if (!futex_cmpxchg_enabled)
-		return;
 	/*
 	 * We are a ZOMBIE and nobody can enqueue itself on
 	 * pi_state_list anymore, but we have to be careful
@@ -1125,26 +1112,6 @@ void futex_exit_release(struct task_struct *tsk)
 	futex_cleanup_end(tsk, FUTEX_STATE_DEAD);
 }
 
-static void __init futex_detect_cmpxchg(void)
-{
-#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
-	u32 curval;
-
-	/*
-	 * This will fail and we want it. Some arch implementations do
-	 * runtime detection of the futex_atomic_cmpxchg_inatomic()
-	 * functionality. We want to know that before we call in any
-	 * of the complex code paths. Also we want to prevent
-	 * registration of robust lists in that case. NULL is
-	 * guaranteed to fault and we get -EFAULT on functional
-	 * implementation, the non-functional ones will return
-	 * -ENOSYS.
-	 */
-	if (futex_cmpxchg_value_locked(&curval, NULL, 0, 0) == -EFAULT)
-		futex_cmpxchg_enabled = 1;
-#endif
-}
-
 static int __init futex_init(void)
 {
 	unsigned int futex_shift;
@@ -1163,8 +1130,6 @@ static int __init futex_init(void)
 					       futex_hashsize, futex_hashsize);
 	futex_hashsize = 1UL << futex_shift;
 
-	futex_detect_cmpxchg();
-
 	for (i = 0; i < futex_hashsize; i++) {
 		atomic_set(&futex_queues[i].waiters, 0);
 		plist_head_init(&futex_queues[i].chain);
diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index 040ae4277cb0..c264cbeab71c 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -27,12 +27,6 @@
 #define FLAGS_CLOCKRT		0x02
 #define FLAGS_HAS_TIMEOUT	0x04
 
-#ifdef CONFIG_HAVE_FUTEX_CMPXCHG
-#define futex_cmpxchg_enabled 1
-#else
-extern int  __read_mostly futex_cmpxchg_enabled;
-#endif
-
 #ifdef CONFIG_FAIL_FUTEX
 extern bool should_fail_futex(bool fshared);
 #else
diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
index 6f91a07a6a83..086a22d1adb7 100644
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -29,8 +29,6 @@
 SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
 		size_t, len)
 {
-	if (!futex_cmpxchg_enabled)
-		return -ENOSYS;
 	/*
 	 * The kernel knows only one size for now:
 	 */
@@ -56,9 +54,6 @@ SYSCALL_DEFINE3(get_robust_list, int, pid,
 	unsigned long ret;
 	struct task_struct *p;
 
-	if (!futex_cmpxchg_enabled)
-		return -ENOSYS;
-
 	rcu_read_lock();
 
 	ret = -ESRCH;
@@ -103,17 +98,6 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 			return -ENOSYS;
 	}
 
-	switch (cmd) {
-	case FUTEX_LOCK_PI:
-	case FUTEX_LOCK_PI2:
-	case FUTEX_UNLOCK_PI:
-	case FUTEX_TRYLOCK_PI:
-	case FUTEX_WAIT_REQUEUE_PI:
-	case FUTEX_CMP_REQUEUE_PI:
-		if (!futex_cmpxchg_enabled)
-			return -ENOSYS;
-	}
-
 	switch (cmd) {
 	case FUTEX_WAIT:
 		val3 = FUTEX_BITSET_MATCH_ANY;
@@ -323,9 +307,6 @@ COMPAT_SYSCALL_DEFINE2(set_robust_list,
 		struct compat_robust_list_head __user *, head,
 		compat_size_t, len)
 {
-	if (!futex_cmpxchg_enabled)
-		return -ENOSYS;
-
 	if (unlikely(len != sizeof(*head)))
 		return -EINVAL;
 
@@ -342,9 +323,6 @@ COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid,
 	unsigned long ret;
 	struct task_struct *p;
 
-	if (!futex_cmpxchg_enabled)
-		return -ENOSYS;
-
 	rcu_read_lock();
 
 	ret = -ESRCH;
-- 
2.29.2

Re: [PATCH 2/2] futex: remove futex_cmpxchg detection

[Geert Uytterhoeven]

On Tue, Oct 26, 2021 at 12:06 PM Arnd Bergmann <arnd@kernel.org> wrote:
> From: Arnd Bergmann <arnd@arndb.de>
>
> Now that all architectures have a working futex implementation
> in any configuration, remove the runtime detection code.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

>  arch/m68k/Kconfig             |  1 -

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH 2/2] futex: remove futex_cmpxchg detection

[Russell King (Oracle)]

On Tue, Oct 26, 2021 at 12:03:48PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> Now that all architectures have a working futex implementation
> in any configuration, remove the runtime detection code.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---

For ARM:

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

As Arnd explained to me what was going on, I'll include it here...
This patch requires patch 1 which touches other architectures, so
it is not an independent patch.

Patch 1 can be found at:
 https://lore.kernel.org/lkml/20211026100432.1730393-1-arnd@kernel.org/T/#t

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH 2/2] futex: remove futex_cmpxchg detection

[Christian Borntraeger]

Am 26.10.21 um 12:03 schrieb Arnd Bergmann:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> Now that all architectures have a working futex implementation
> in any configuration, remove the runtime detection code.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

s390 part
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>

> ---
>   arch/arc/Kconfig              |  1 -
>   arch/arm/Kconfig              |  1 -
>   arch/arm64/Kconfig            |  1 -
>   arch/csky/Kconfig             |  1 -
>   arch/m68k/Kconfig             |  1 -
>   arch/riscv/Kconfig            |  1 -
>   arch/s390/Kconfig             |  1 -
>   arch/sh/Kconfig               |  1 -
>   arch/um/Kconfig               |  1 -
>   arch/um/kernel/skas/uaccess.c |  1 -
>   arch/xtensa/Kconfig           |  1 -
>   init/Kconfig                  |  8 --------
>   kernel/futex/core.c           | 35 -----------------------------------
>   kernel/futex/futex.h          |  6 ------
>   kernel/futex/syscalls.c       | 22 ----------------------
>   15 files changed, 82 deletions(-)
> 
> diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
> index 248389278e8f..f9413041686f 100644
> --- a/arch/arc/Kconfig
> +++ b/arch/arc/Kconfig
> @@ -31,7 +31,6 @@ config ARC
>   	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARC_MMU_V4
>   	select HAVE_DEBUG_STACKOVERFLOW
>   	select HAVE_DEBUG_KMEMLEAK
> -	select HAVE_FUTEX_CMPXCHG if FUTEX
>   	select HAVE_IOREMAP_PROT
>   	select HAVE_KERNEL_GZIP
>   	select HAVE_KERNEL_LZMA
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index bb5d2c45477b..6448d311635d 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -93,7 +93,6 @@ config ARM
>   	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
>   	select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
>   	select HAVE_FUNCTION_TRACER if !XIP_KERNEL && !(THUMB2_KERNEL && CC_IS_CLANG)
> -	select HAVE_FUTEX_CMPXCHG if FUTEX
>   	select HAVE_GCC_PLUGINS
>   	select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
>   	select HAVE_IRQ_TIME_ACCOUNTING
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 0efc501f77aa..6c3c2ff5cef8 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -193,7 +193,6 @@ config ARM64
>   	select HAVE_PERF_USER_STACK_DUMP
>   	select HAVE_REGS_AND_STACK_ACCESS_API
>   	select HAVE_FUNCTION_ARG_ACCESS_API
> -	select HAVE_FUTEX_CMPXCHG if FUTEX
>   	select MMU_GATHER_RCU_TABLE_FREE
>   	select HAVE_RSEQ
>   	select HAVE_STACKPROTECTOR
> diff --git a/arch/csky/Kconfig b/arch/csky/Kconfig
> index 823d3d5a9e11..efd7c5feac8b 100644
> --- a/arch/csky/Kconfig
> +++ b/arch/csky/Kconfig
> @@ -53,7 +53,6 @@ config CSKY
>   	select HAVE_FUNCTION_TRACER
>   	select HAVE_FUNCTION_GRAPH_TRACER
>   	select HAVE_FUNCTION_ERROR_INJECTION
> -	select HAVE_FUTEX_CMPXCHG if FUTEX && SMP
>   	select HAVE_FTRACE_MCOUNT_RECORD
>   	select HAVE_KERNEL_GZIP
>   	select HAVE_KERNEL_LZO
> diff --git a/arch/m68k/Kconfig b/arch/m68k/Kconfig
> index 0b50da08a9c5..15a793c5b2dc 100644
> --- a/arch/m68k/Kconfig
> +++ b/arch/m68k/Kconfig
> @@ -20,7 +20,6 @@ config M68K
>   	select HAVE_ASM_MODVERSIONS
>   	select HAVE_DEBUG_BUGVERBOSE
>   	select HAVE_EFFICIENT_UNALIGNED_ACCESS if !CPU_HAS_NO_UNALIGNED
> -	select HAVE_FUTEX_CMPXCHG if MMU && FUTEX
>   	select HAVE_MOD_ARCH_SPECIFIC
>   	select HAVE_UID16
>   	select MMU_GATHER_NO_RANGE if MMU
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 77a088d0a7e9..037fea9fac14 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -84,7 +84,6 @@ config RISCV
>   	select HAVE_DMA_CONTIGUOUS if MMU
>   	select HAVE_EBPF_JIT if MMU
>   	select HAVE_FUNCTION_ERROR_INJECTION
> -	select HAVE_FUTEX_CMPXCHG if FUTEX
>   	select HAVE_GCC_PLUGINS
>   	select HAVE_GENERIC_VDSO if MMU && 64BIT
>   	select HAVE_IRQ_TIME_ACCOUNTING
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index f615c3f65f5a..1c9ecf619e04 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -164,7 +164,6 @@ config S390
>   	select HAVE_FUNCTION_ERROR_INJECTION
>   	select HAVE_FUNCTION_GRAPH_TRACER
>   	select HAVE_FUNCTION_TRACER
> -	select HAVE_FUTEX_CMPXCHG if FUTEX
>   	select HAVE_GCC_PLUGINS
>   	select HAVE_GENERIC_VDSO
>   	select HAVE_IOREMAP_PROT if PCI
> diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
> index 6904f4bdbf00..93195d3368c0 100644
> --- a/arch/sh/Kconfig
> +++ b/arch/sh/Kconfig
> @@ -34,7 +34,6 @@ config SUPERH
>   	select HAVE_FAST_GUP if MMU
>   	select HAVE_FUNCTION_GRAPH_TRACER
>   	select HAVE_FUNCTION_TRACER
> -	select HAVE_FUTEX_CMPXCHG if FUTEX
>   	select HAVE_FTRACE_MCOUNT_RECORD
>   	select HAVE_HW_BREAKPOINT
>   	select HAVE_IOREMAP_PROT if MMU && !X2TLB
> diff --git a/arch/um/Kconfig b/arch/um/Kconfig
> index c18b45f75d41..c906250d4970 100644
> --- a/arch/um/Kconfig
> +++ b/arch/um/Kconfig
> @@ -14,7 +14,6 @@ config UML
>   	select HAVE_ARCH_SECCOMP_FILTER
>   	select HAVE_ASM_MODVERSIONS
>   	select HAVE_UID16
> -	select HAVE_FUTEX_CMPXCHG if FUTEX
>   	select HAVE_DEBUG_KMEMLEAK
>   	select HAVE_DEBUG_BUGVERBOSE
>   	select NO_DMA if !UML_DMA_EMULATION
> diff --git a/arch/um/kernel/skas/uaccess.c b/arch/um/kernel/skas/uaccess.c
> index a509be911026..9e37a7c05990 100644
> --- a/arch/um/kernel/skas/uaccess.c
> +++ b/arch/um/kernel/skas/uaccess.c
> @@ -348,7 +348,6 @@ EXPORT_SYMBOL(arch_futex_atomic_op_inuser);
>    * 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)
>    */
>   
>   int futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
> diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
> index 0e56bad058fa..8ac599aa6d99 100644
> --- a/arch/xtensa/Kconfig
> +++ b/arch/xtensa/Kconfig
> @@ -31,7 +31,6 @@ config XTENSA
>   	select HAVE_DMA_CONTIGUOUS
>   	select HAVE_EXIT_THREAD
>   	select HAVE_FUNCTION_TRACER
> -	select HAVE_FUTEX_CMPXCHG if !MMU && FUTEX
>   	select HAVE_HW_BREAKPOINT if PERF_EVENTS
>   	select HAVE_IRQ_TIME_ACCOUNTING
>   	select HAVE_PCI
> diff --git a/init/Kconfig b/init/Kconfig
> index c0f55ea5a71f..538688598f2f 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1597,14 +1597,6 @@ config FUTEX_PI
>   	depends on FUTEX && RT_MUTEXES
>   	default y
>   
> -config HAVE_FUTEX_CMPXCHG
> -	bool
> -	depends on FUTEX
> -	help
> -	  Architectures should select this if futex_atomic_cmpxchg_inatomic()
> -	  is implemented and always working. This removes a couple of runtime
> -	  checks.
> -
>   config EPOLL
>   	bool "Enable eventpoll support" if EXPERT
>   	default y
> diff --git a/kernel/futex/core.c b/kernel/futex/core.c
> index 25d8a88b32e5..926c2bb752bc 100644
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -41,11 +41,6 @@
>   #include "futex.h"
>   #include "../locking/rtmutex_common.h"
>   
> -#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
> -int  __read_mostly futex_cmpxchg_enabled;
> -#endif
> -
> -
>   /*
>    * The base of the bucket array and its size are always used together
>    * (after initialization only in futex_hash()), so ensure that they
> @@ -776,9 +771,6 @@ static void exit_robust_list(struct task_struct *curr)
>   	unsigned long futex_offset;
>   	int rc;
>   
> -	if (!futex_cmpxchg_enabled)
> -		return;
> -
>   	/*
>   	 * Fetch the list head (which was registered earlier, via
>   	 * sys_set_robust_list()):
> @@ -874,9 +866,6 @@ static void compat_exit_robust_list(struct task_struct *curr)
>   	compat_long_t futex_offset;
>   	int rc;
>   
> -	if (!futex_cmpxchg_enabled)
> -		return;
> -
>   	/*
>   	 * Fetch the list head (which was registered earlier, via
>   	 * sys_set_robust_list()):
> @@ -950,8 +939,6 @@ static void exit_pi_state_list(struct task_struct *curr)
>   	struct futex_hash_bucket *hb;
>   	union futex_key key = FUTEX_KEY_INIT;
>   
> -	if (!futex_cmpxchg_enabled)
> -		return;
>   	/*
>   	 * We are a ZOMBIE and nobody can enqueue itself on
>   	 * pi_state_list anymore, but we have to be careful
> @@ -1125,26 +1112,6 @@ void futex_exit_release(struct task_struct *tsk)
>   	futex_cleanup_end(tsk, FUTEX_STATE_DEAD);
>   }
>   
> -static void __init futex_detect_cmpxchg(void)
> -{
> -#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
> -	u32 curval;
> -
> -	/*
> -	 * This will fail and we want it. Some arch implementations do
> -	 * runtime detection of the futex_atomic_cmpxchg_inatomic()
> -	 * functionality. We want to know that before we call in any
> -	 * of the complex code paths. Also we want to prevent
> -	 * registration of robust lists in that case. NULL is
> -	 * guaranteed to fault and we get -EFAULT on functional
> -	 * implementation, the non-functional ones will return
> -	 * -ENOSYS.
> -	 */
> -	if (futex_cmpxchg_value_locked(&curval, NULL, 0, 0) == -EFAULT)
> -		futex_cmpxchg_enabled = 1;
> -#endif
> -}
> -
>   static int __init futex_init(void)
>   {
>   	unsigned int futex_shift;
> @@ -1163,8 +1130,6 @@ static int __init futex_init(void)
>   					       futex_hashsize, futex_hashsize);
>   	futex_hashsize = 1UL << futex_shift;
>   
> -	futex_detect_cmpxchg();
> -
>   	for (i = 0; i < futex_hashsize; i++) {
>   		atomic_set(&futex_queues[i].waiters, 0);
>   		plist_head_init(&futex_queues[i].chain);
> diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
> index 040ae4277cb0..c264cbeab71c 100644
> --- a/kernel/futex/futex.h
> +++ b/kernel/futex/futex.h
> @@ -27,12 +27,6 @@
>   #define FLAGS_CLOCKRT		0x02
>   #define FLAGS_HAS_TIMEOUT	0x04
>   
> -#ifdef CONFIG_HAVE_FUTEX_CMPXCHG
> -#define futex_cmpxchg_enabled 1
> -#else
> -extern int  __read_mostly futex_cmpxchg_enabled;
> -#endif
> -
>   #ifdef CONFIG_FAIL_FUTEX
>   extern bool should_fail_futex(bool fshared);
>   #else
> diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
> index 6f91a07a6a83..086a22d1adb7 100644
> --- a/kernel/futex/syscalls.c
> +++ b/kernel/futex/syscalls.c
> @@ -29,8 +29,6 @@
>   SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
>   		size_t, len)
>   {
> -	if (!futex_cmpxchg_enabled)
> -		return -ENOSYS;
>   	/*
>   	 * The kernel knows only one size for now:
>   	 */
> @@ -56,9 +54,6 @@ SYSCALL_DEFINE3(get_robust_list, int, pid,
>   	unsigned long ret;
>   	struct task_struct *p;
>   
> -	if (!futex_cmpxchg_enabled)
> -		return -ENOSYS;
> -
>   	rcu_read_lock();
>   
>   	ret = -ESRCH;
> @@ -103,17 +98,6 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
>   			return -ENOSYS;
>   	}
>   
> -	switch (cmd) {
> -	case FUTEX_LOCK_PI:
> -	case FUTEX_LOCK_PI2:
> -	case FUTEX_UNLOCK_PI:
> -	case FUTEX_TRYLOCK_PI:
> -	case FUTEX_WAIT_REQUEUE_PI:
> -	case FUTEX_CMP_REQUEUE_PI:
> -		if (!futex_cmpxchg_enabled)
> -			return -ENOSYS;
> -	}
> -
>   	switch (cmd) {
>   	case FUTEX_WAIT:
>   		val3 = FUTEX_BITSET_MATCH_ANY;
> @@ -323,9 +307,6 @@ COMPAT_SYSCALL_DEFINE2(set_robust_list,
>   		struct compat_robust_list_head __user *, head,
>   		compat_size_t, len)
>   {
> -	if (!futex_cmpxchg_enabled)
> -		return -ENOSYS;
> -
>   	if (unlikely(len != sizeof(*head)))
>   		return -EINVAL;
>   
> @@ -342,9 +323,6 @@ COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid,
>   	unsigned long ret;
>   	struct task_struct *p;
>   
> -	if (!futex_cmpxchg_enabled)
> -		return -ENOSYS;
> -
>   	rcu_read_lock();
>   
>   	ret = -ESRCH;
> 

Re: [PATCH 2/2] futex: remove futex_cmpxchg detection

[Max Filippov]

On Tue, Oct 26, 2021 at 3:06 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> Now that all architectures have a working futex implementation
> in any configuration, remove the runtime detection code.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  arch/arc/Kconfig              |  1 -
>  arch/arm/Kconfig              |  1 -
>  arch/arm64/Kconfig            |  1 -
>  arch/csky/Kconfig             |  1 -
>  arch/m68k/Kconfig             |  1 -
>  arch/riscv/Kconfig            |  1 -
>  arch/s390/Kconfig             |  1 -
>  arch/sh/Kconfig               |  1 -
>  arch/um/Kconfig               |  1 -
>  arch/um/kernel/skas/uaccess.c |  1 -
>  arch/xtensa/Kconfig           |  1 -
>  init/Kconfig                  |  8 --------
>  kernel/futex/core.c           | 35 -----------------------------------
>  kernel/futex/futex.h          |  6 ------
>  kernel/futex/syscalls.c       | 22 ----------------------
>  15 files changed, 82 deletions(-)

For xtensa:
Acked-by: Max Filippov <jcmvbkbc@gmail.com>

-- 
Thanks.
-- Max

Re: [PATCH 2/2] futex: remove futex_cmpxchg detection

[Rich Felker]

On Tue, Oct 26, 2021 at 12:03:48PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> Now that all architectures have a working futex implementation
> in any configuration, remove the runtime detection code.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  arch/arc/Kconfig              |  1 -
>  arch/arm/Kconfig              |  1 -
>  arch/arm64/Kconfig            |  1 -
>  arch/csky/Kconfig             |  1 -
>  arch/m68k/Kconfig             |  1 -
>  arch/riscv/Kconfig            |  1 -
>  arch/s390/Kconfig             |  1 -
>  arch/sh/Kconfig               |  1 -
>  arch/um/Kconfig               |  1 -
>  arch/um/kernel/skas/uaccess.c |  1 -
>  arch/xtensa/Kconfig           |  1 -
>  init/Kconfig                  |  8 --------
>  kernel/futex/core.c           | 35 -----------------------------------
>  kernel/futex/futex.h          |  6 ------
>  kernel/futex/syscalls.c       | 22 ----------------------
>  15 files changed, 82 deletions(-)

Acked-by: Rich Felker <dalias@libc.org>