rseq: Implement sys_rseq_slice_yield()

Provide a new syscall which has the only purpose to yield the CPU after the
kernel granted a time slice extension.

sched_yield() is not suitable for that because it unconditionally
schedules, but the end of the time slice extension is not required to
schedule when the task was already preempted. This also allows to have a
strict check for termination to catch user space invoking random syscalls
including sched_yield() from a time slice extension region.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20251215155708.929634896@linutronix.de

arch/alpha/kernel/syscalls/syscall.tbl

@@ -510,3 +510,4 @@
 578	common	file_getattr			sys_file_getattr
 579	common	file_setattr			sys_file_setattr
 580	common	listns				sys_listns
+581	common	rseq_slice_yield		sys_rseq_slice_yield

arch/arm/tools/syscall.tbl

@@ -485,3 +485,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield

arch/arm64/tools/syscall_32.tbl

@@ -482,3 +482,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield

arch/m68k/kernel/syscalls/syscall.tbl

@@ -470,3 +470,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield

arch/microblaze/kernel/syscalls/syscall.tbl

@@ -476,3 +476,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield

arch/mips/kernel/syscalls/syscall_n32.tbl

@@ -409,3 +409,4 @@
 468	n32	file_getattr			sys_file_getattr
 469	n32	file_setattr			sys_file_setattr
 470	n32	listns				sys_listns
+471	n32	rseq_slice_yield		sys_rseq_slice_yield

arch/mips/kernel/syscalls/syscall_n64.tbl

@@ -385,3 +385,4 @@
 468	n64	file_getattr			sys_file_getattr
 469	n64	file_setattr			sys_file_setattr
 470	n64	listns				sys_listns
+471	n64	rseq_slice_yield		sys_rseq_slice_yield

arch/mips/kernel/syscalls/syscall_o32.tbl

@@ -458,3 +458,4 @@
 468	o32	file_getattr			sys_file_getattr
 469	o32	file_setattr			sys_file_setattr
 470	o32	listns				sys_listns
+471	o32	rseq_slice_yield		sys_rseq_slice_yield

arch/parisc/kernel/syscalls/syscall.tbl

@@ -469,3 +469,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield

arch/powerpc/kernel/syscalls/syscall.tbl

@@ -561,3 +561,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	nospu	rseq_slice_yield		sys_rseq_slice_yield

arch/s390/kernel/syscalls/syscall.tbl

@@ -397,3 +397,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield

arch/sh/kernel/syscalls/syscall.tbl

@@ -474,3 +474,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield

arch/sparc/kernel/syscalls/syscall.tbl

@@ -516,3 +516,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield

arch/x86/entry/syscalls/syscall_32.tbl

@@ -476,3 +476,4 @@
 468	i386	file_getattr		sys_file_getattr
 469	i386	file_setattr		sys_file_setattr
 470	i386	listns			sys_listns
+471	i386	rseq_slice_yield	sys_rseq_slice_yield

arch/x86/entry/syscalls/syscall_64.tbl

@@ -395,6 +395,7 @@
 468	common	file_getattr		sys_file_getattr
 469	common	file_setattr		sys_file_setattr
 470	common	listns			sys_listns
+471	common	rseq_slice_yield	sys_rseq_slice_yield
 
 #
 # Due to a historical design error, certain syscalls are numbered differently

arch/xtensa/kernel/syscalls/syscall.tbl

@@ -441,3 +441,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield

include/linux/rseq_types.h

@@ -89,9 +89,11 @@ union rseq_slice_state {
 /**
  * struct rseq_slice - Status information for rseq time slice extension
  * @state:	Time slice extension state
+ * @yielded:	Indicator for rseq_slice_yield()
  */
 struct rseq_slice {
 	union rseq_slice_state	state;
+	u8			yielded;
 };
 
 /**

include/linux/syscalls.h

@@ -961,6 +961,7 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
 			  unsigned mask, struct statx __user *buffer);
 asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
 			 int flags, uint32_t sig);
+asmlinkage long sys_rseq_slice_yield(void);
 asmlinkage long sys_open_tree(int dfd, const char __user *path, unsigned flags);
 asmlinkage long sys_open_tree_attr(int dfd, const char __user *path,
 				   unsigned flags,

include/uapi/asm-generic/unistd.h

@@ -860,8 +860,11 @@ __SYSCALL(__NR_file_setattr, sys_file_setattr)
 #define __NR_listns 470
 __SYSCALL(__NR_listns, sys_listns)
 
+#define __NR_rseq_slice_yield 471
+__SYSCALL(__NR_rseq_slice_yield, sys_rseq_slice_yield)
+
 #undef __NR_syscalls
-#define __NR_syscalls 471
+#define __NR_syscalls 472
 
 /*
  * 32 bit systems traditionally used different

kernel/rseq.c

@@ -553,6 +553,27 @@ die:
 	return -EFAULT;
 }
 
+/**
+ * sys_rseq_slice_yield - yield the current processor side effect free if a
+ *			  task granted with a time slice extension is done with
+ *			  the critical work before being forced out.
+ *
+ * Return: 1 if the task successfully yielded the CPU within the granted slice.
+ *         0 if the slice extension was either never granted or was revoked by
+ *	     going over the granted extension, using a syscall other than this one
+ *	     or being scheduled out earlier due to a subsequent interrupt.
+ *
+ * The syscall does not schedule because the syscall entry work immediately
+ * relinquishes the CPU and schedules if required.
+ */
+SYSCALL_DEFINE0(rseq_slice_yield)
+{
+	int yielded = !!current->rseq.slice.yielded;
+
+	current->rseq.slice.yielded = 0;
+	return yielded;
+}
+
 static int __init rseq_slice_cmdline(char *str)
 {
 	bool on;

kernel/sys_ni.c

@@ -390,6 +390,7 @@ COND_SYSCALL(setuid16);
 
 /* restartable sequence */
 COND_SYSCALL(rseq);
+COND_SYSCALL(rseq_slice_yield);
 
 COND_SYSCALL(uretprobe);
 COND_SYSCALL(uprobe);

scripts/syscall.tbl

@@ -411,3 +411,4 @@
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
 470	common	listns				sys_listns
+471	common	rseq_slice_yield		sys_rseq_slice_yield