Index: lib/libc/dlfcn/init.c
===================================================================
RCS file: /cvs/src/lib/libc/dlfcn/init.c,v
diff -u -p -r1.24 init.c
--- lib/libc/dlfcn/init.c	22 Jul 2024 22:06:27 -0000	1.24
+++ lib/libc/dlfcn/init.c	2 Oct 2024 21:03:02 -0000
@@ -21,6 +21,7 @@
 #include <sys/types.h>
 #include <sys/syscall.h>
 #include <sys/timetc.h>		/* timekeep */
+#include <sys/sysctl.h>		/* ncpus */
 
 #ifndef PIC
 #include <sys/mman.h>
@@ -51,6 +52,7 @@ int	_pagesize = 0;
 struct timekeep	*_timekeep;
 unsigned long	_hwcap, _hwcap2;
 int	_hwcap_avail, _hwcap2_avail;
+int	_ncpus;
 
 /*
  * In dynamically linked binaries environ and __progname are overridden by
@@ -130,6 +132,9 @@ _libc_preinit(int argc, char **argv, cha
 			if (issetugid() == 0 && getenv("LIBC_NOUSERTC"))
 				_timekeep = NULL;
 			break;
+		case AUX_openbsd_ncpus:
+			_ncpus = aux->au_v;
+			break;
 		}
 	}
 
@@ -167,6 +172,14 @@ _libc_preinit(int argc, char **argv, cha
 		}
 	}
 #endif /* !PIC */
+
+	if (_ncpus == 0) {
+		int mib[] = { CTL_HW, HW_NCPU };
+		size_t len = sizeof(_ncpus);
+
+		(void)sysctl(mib, sizeof(mib) / sizeof(mib[0]),
+		    &_ncpus, &len, NULL, 0);
+	}
 }
 
 /* ARM just had to be different... */
Index: lib/libc/gen/sysconf.c
===================================================================
RCS file: /cvs/src/lib/libc/gen/sysconf.c,v
diff -u -p -r1.28 sysconf.c
--- lib/libc/gen/sysconf.c	19 Jul 2022 09:25:44 -0000	1.28
+++ lib/libc/gen/sysconf.c	2 Oct 2024 21:03:02 -0000
@@ -45,6 +45,8 @@
 #include <stdio.h>
 #include <unistd.h>
 
+extern int _ncpus; /* passed by the kernel as an aux vector value */
+
 /*
  * sysconf --
  *	get configurable system variables.
@@ -461,6 +463,8 @@ sysconf(int name)
 	}
 
 	case _SC_NPROCESSORS_CONF:
+		if (_ncpus != 0)
+			return (_ncpus);
 		mib[0] = CTL_HW;
 		mib[1] = HW_NCPU;
 		break;
Index: lib/libc/include/thread_private.h
===================================================================
RCS file: /cvs/src/lib/libc/include/thread_private.h,v
diff -u -p -r1.37 thread_private.h
--- lib/libc/include/thread_private.h	18 Aug 2024 02:25:51 -0000	1.37
+++ lib/libc/include/thread_private.h	2 Oct 2024 21:03:02 -0000
@@ -6,6 +6,7 @@
 #define _THREAD_PRIVATE_H_
 
 extern int __isthreaded;
+extern int _ncpus;
 
 #define _MALLOC_MUTEXES 32
 void _malloc_init(int);
Index: lib/libc/thread/rthread.c
===================================================================
RCS file: /cvs/src/lib/libc/thread/rthread.c,v
diff -u -p -r1.9 rthread.c
--- lib/libc/thread/rthread.c	12 Oct 2020 22:06:51 -0000	1.9
+++ lib/libc/thread/rthread.c	2 Oct 2024 21:03:02 -0000
@@ -43,11 +46,43 @@ struct pthread _initial_thread = {
 /*
  * internal support functions
  */
+
+/*
+ * Wait for the spinlock to become unlocked.
+ *
+ * On uniprocessor systems it is pointless to spin waiting for
+ * another thread to release the lock because this thread occupies
+ * the only CPU, preventing the thread holding the lock from running
+ * and leaving the critical section.
+ *
+ * On multiprocessor systems we spin, but not forever in case there
+ * are more threads than CPUs still, and more progress might be made
+ * if we can get the other thread to run.
+ */
+
+static inline void
+_spinlock_wait(volatile _atomic_lock_t *lock)
+{
+	do {
+		if (_ncpus > 1) {
+			unsigned int spin;
+
+			for (spin = 0; spin < SPIN_COUNT; spin++) {
+				SPIN_WAIT();
+				if (*lock == _ATOMIC_LOCK_UNLOCKED)
+					return;
+			}
+		}
+
+		sched_yield();
+	} while (*lock != _ATOMIC_LOCK_UNLOCKED);
+}
+
 void
 _spinlock(volatile _atomic_lock_t *lock)
 {
 	while (_atomic_lock(lock))
-		sched_yield();
+		_spinlock_wait(lock);
 	membar_enter_after_atomic();
 }
 DEF_STRONG(_spinlock);
Index: sys/kern/exec_elf.c
===================================================================
RCS file: /cvs/src/sys/kern/exec_elf.c,v
diff -u -p -r1.191 exec_elf.c
--- sys/kern/exec_elf.c	15 Sep 2024 23:13:19 -0000	1.191
+++ sys/kern/exec_elf.c	2 Oct 2024 21:03:02 -0000
@@ -1023,6 +1023,10 @@ exec_elf_fixup(struct proc *p, struct ex
 		a->au_v = p->p_p->ps_timekeep;
 		a++;
 
+		a->au_id = AUX_openbsd_ncpus;
+		a->au_v = ncpus;
+		a++;
+
 		a->au_id = AUX_null;
 		a->au_v = 0;
 		a++;
Index: sys/sys/exec_elf.h
===================================================================
RCS file: /cvs/src/sys/sys/exec_elf.h,v
diff -u -p -r1.105 exec_elf.h
--- sys/sys/exec_elf.h	14 Jul 2024 09:48:49 -0000	1.105
+++ sys/sys/exec_elf.h	2 Oct 2024 21:03:02 -0000
@@ -734,6 +734,7 @@ enum AuxID {
 	AUX_sun_gid = 2002,		/* egid */
 	AUX_sun_rgid = 2003,		/* rgid */
 	AUX_openbsd_timekeep = 4000,	/* userland clock_gettime */
+	AUX_openbsd_ncpus = 4001,	/* ncpus */
 };
 
 struct elf_args {
@@ -822,7 +823,7 @@ extern Elf_Dyn		_DYNAMIC[];
 /*
  * How many entries are in the AuxInfo array we pass to the process?
  */
-#define	ELF_AUX_ENTRIES	11
+#define	ELF_AUX_ENTRIES	12
 #define	ELF_AUX_WORDS	(sizeof(AuxInfo) * ELF_AUX_ENTRIES / sizeof(char *))
 
 struct exec_package;