futex2: Implement wait and wake functions

Create a new set of futex syscalls known as futex2. This new interface
is aimed to implement a more maintainable code, while removing obsolete
features and expanding it with new functionalities.

Implements wait and wake semantics for futexes, along with the base
infrastructure for future operations. The whole wait path is designed to
be used by N waiters, thus making easier to implement vectorized wait.

* Syscalls implemented by this patch:

- futex_wait(void *uaddr, unsigned int val, unsigned int flags,
	     struct timespec *timo)

   The user thread is put to sleep, waiting for a futex_wake() at uaddr,
   if the value at *uaddr is the same as val (otherwise, the syscall
   returns immediately with -EAGAIN). timo is an optional timeout value
   for the operation.

   Return 0 on success, error code otherwise.

 - futex_wake(void *uaddr, unsigned long nr_wake, unsigned int flags)

   Wake `nr_wake` threads waiting at uaddr.

   Return the number of woken threads on success, error code otherwise.

** The `flag` argument

 The flag is used to specify the size of the futex word
 (FUTEX_[8, 16, 32]). It's mandatory to define one, since there's no
 default size.

 By default, the timeout uses a monotonic clock, but can be used as a
 realtime one by using the FUTEX_REALTIME_CLOCK flag.

 By default, futexes are of the private type, that means that this user
 address will be accessed by threads that shares the same memory region.
 This allows for some internal optimizations, so they are faster.
 However, if the address needs to be shared with different processes
 (like using `mmap()` or `shm()`), they need to be defined as shared and
 the flag FUTEX_SHARED_FLAG is used to set that.

 By default, the operation has no NUMA-awareness, meaning that the user
 can't choose the memory node where the kernel side futex data will be
 stored. The user can choose the node where it wants to operate by
 setting the FUTEX_NUMA_FLAG and using the following structure (where X
 can be 8, 16, or 32):

  struct futexX_numa {
          __uX value;
          __sX hint;
  };

 This structure should be passed at the `void *uaddr` of futex
 functions. The address of the structure will be used to be waited/waken
 on, and the `value` will be compared to `val` as usual. The `hint`
 member is used to defined which node the futex will use. When waiting,
 the futex will be registered on a kernel-side table stored on that
 node; when waking, the futex will be searched for on that given table.
 That means that there's no redundancy between tables, and the wrong
 `hint` value will led to undesired behavior.  Userspace is responsible
 for dealing with node migrations issues that may occur. `hint` can
 range from [0, MAX_NUMA_NODES], for specifying a node, or -1, to use
 the same node the current process is using.

 When not using FUTEX_NUMA_FLAG on a NUMA system, the futex will be
 stored on a global table on some node, defined at compilation time.

** The `timo` argument

As per the Y2038 work done in the kernel, new interfaces shouldn't add
timeout options known to be buggy. Given that, `timo` should be a 64bit
timeout at all platforms, using an absolute timeout value.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Alexandre Frade <kernel@xanmod.org>

MAINTAINERS

@@ -7332,7 +7332,7 @@ F:	Documentation/locking/*futex*
 F:	include/asm-generic/futex.h
 F:	include/linux/futex.h
 F:	include/uapi/linux/futex.h
-F:	kernel/futex.c
+F:	kernel/futex*
 F:	tools/perf/bench/futex*
 F:	tools/testing/selftests/futex/
 

arch/arm/tools/syscall.tbl

@@ -455,3 +455,5 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	futex_wait			sys_futex_wait
+443	common	futex_wake			sys_futex_wake

arch/arm64/include/asm/unistd.h

@@ -38,7 +38,7 @@
 #define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
 #define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
 
-#define __NR_compat_syscalls		442
+#define __NR_compat_syscalls		444
 #endif
 
 #define __ARCH_WANT_SYS_CLONE

arch/arm64/include/asm/unistd32.h

@@ -891,6 +891,10 @@ __SYSCALL(__NR_faccessat2, sys_faccessat2)
 __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SYSCALL(__NR_epoll_pwait2, compat_sys_epoll_pwait2)
+#define __NR_futex_wait 442
+__SYSCALL(__NR_futex_wait, sys_futex_wait)
+#define __NR_futex_wake 443
+__SYSCALL(__NR_futex_wake, sys_futex_wake)
 
 /*
  * Please add new compat syscalls above this comment and update

arch/x86/entry/syscalls/syscall_32.tbl

@@ -446,3 +446,5 @@
 439	i386	faccessat2		sys_faccessat2
 440	i386	process_madvise		sys_process_madvise
 441	i386	epoll_pwait2		sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	i386	futex_wait		sys_futex_wait
+443	i386	futex_wake		sys_futex_wake

arch/x86/entry/syscalls/syscall_64.tbl

@@ -363,6 +363,8 @@
 439	common	faccessat2		sys_faccessat2
 440	common	process_madvise		sys_process_madvise
 441	common	epoll_pwait2		sys_epoll_pwait2
+442	common	futex_wait		sys_futex_wait
+443	common	futex_wake		sys_futex_wake
 
 #
 # Due to a historical design error, certain syscalls are numbered differently

include/linux/syscalls.h

@@ -618,6 +618,13 @@ asmlinkage long sys_get_robust_list(int pid,
 asmlinkage long sys_set_robust_list(struct robust_list_head __user *head,
 				    size_t len);
 
+/* kernel/futex2.c */
+asmlinkage long sys_futex_wait(void __user *uaddr, unsigned int val,
+			       unsigned int flags,
+			       struct __kernel_timespec __user __user *timo);
+asmlinkage long sys_futex_wake(void __user *uaddr, unsigned int nr_wake,
+			       unsigned int flags);
+
 /* kernel/hrtimer.c */
 asmlinkage long sys_nanosleep(struct __kernel_timespec __user *rqtp,
 			      struct __kernel_timespec __user *rmtp);

include/uapi/asm-generic/unistd.h

@@ -862,8 +862,14 @@ __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SC_COMP(__NR_epoll_pwait2, sys_epoll_pwait2, compat_sys_epoll_pwait2)
 
+#define __NR_futex_wait 442
+__SYSCALL(__NR_futex_wait, sys_futex_wait)
+
+#define __NR_futex_wake 443
+__SYSCALL(__NR_futex_wake, sys_futex_wake)
+
 #undef __NR_syscalls
-#define __NR_syscalls 442
+#define __NR_syscalls 444
 
 /*
  * 32 bit systems traditionally used different

include/uapi/linux/futex.h

@@ -44,6 +44,62 @@
 #define FUTEX_WAIT_MULTIPLE_PRIVATE	(FUTEX_WAIT_MULTIPLE | \
 					 FUTEX_PRIVATE_FLAG)
 
+/* Size argument to futex2 syscall */
+#define FUTEX_8		0
+#define FUTEX_16	1
+#define FUTEX_32	2
+
+#define FUTEX_SIZE_MASK	0x3
+
+#define FUTEX_SHARED_FLAG 8
+
+#define FUTEX_NUMA_FLAG 16
+
+/**
+ * struct futexXX_numa - struct for NUMA-aware futex operation
+ * @value: futex value
+ * @hint:  node id to operate
+ */
+
+struct futex8_numa {
+	__u8 value;
+	__s8 hint;
+};
+
+struct futex16_numa {
+	__u16 value;
+	__s16 hint;
+};
+
+struct futex32_numa {
+	__u32 value;
+	__s32 hint;
+};
+
+#define FUTEX_WAITV_MAX 128
+
+/**
+ * struct futex_waitv - A waiter for vectorized wait
+ * @uaddr: User address to wait on
+ * @val:   Expected value at uaddr
+ * @flags: Flags for this waiter
+ */
+struct futex_waitv {
+	void *uaddr;
+	unsigned int val;
+	unsigned int flags;
+};
+
+/**
+ * struct futex_requeue - Define an address and its flags for requeue operation
+ * @uaddr: User address of one of the requeue arguments
+ * @flags: Flags for this address
+ */
+struct futex_requeue {
+	void *uaddr;
+	unsigned int flags;
+};
+
 /*
  * Support for robust futexes: the kernel cleans up held futexes at
  * thread exit time.

init/Kconfig

@@ -1543,6 +1543,13 @@ config FUTEX
 	  support for "fast userspace mutexes".  The resulting kernel may not
 	  run glibc-based applications correctly.
 
+config FUTEX2
+	bool "Enable futex2 support" if EXPERT
+	depends on FUTEX
+	default y
+	help
+	  Support for futex2 interface.
+
 config FUTEX_PI
 	bool
 	depends on FUTEX && RT_MUTEXES

kernel/Makefile

@@ -57,6 +57,7 @@ obj-$(CONFIG_PROFILING) += profile.o
 obj-$(CONFIG_STACKTRACE) += stacktrace.o
 obj-y += time/
 obj-$(CONFIG_FUTEX) += futex.o
+obj-$(CONFIG_FUTEX2) += futex2.o
 obj-$(CONFIG_GENERIC_ISA_DMA) += dma.o
 obj-$(CONFIG_SMP) += smp.o
 ifneq ($(CONFIG_SMP),y)