runtime: respect timeout in semasleep on Darwin

semasleep on Darwin was refactored in https://golang.org/cl/118736 to
use the pthread_cond_timedwait function from libc. The new code
incorrectly assumed that pthread_cond_timedwait took a timeout relative
to the current time, when it in fact it takes a timeout specified in
absolute time. semasleep thus specified a timeout well in the past,
causing it to immediately exceed the timeout and spin hot. This was the
source of a large performance hit to CockroachDB (#26019).

Adjust semasleep to instead call pthread_cond_timedwait_relative_np,
which properly interprets its timeout parameter as relative to the
current time.

pthread_cond_timedwait_relative_np is non-portable, but using
pthread_cond_timedwait correctly would require two calls to
gettimeofday: one in the runtime package to convert the relative timeout
to absolute time, then another in the pthread library to convert back to
a relative offset [0], as the Darwin kernel expects a relative offset.

[0]: https://opensource.apple.com/source/libpthread/libpthread-301.30.1/src/pthread_cond.c.auto.html

Fix #26019.

Change-Id: I1a8c2429f79513b43d2b256365cd9166d235af8b
Reviewed-on: https://go-review.googlesource.com/120635Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/os_darwin.go

@@ -45,7 +45,7 @@ func semasleep(ns int64) int32 {
 		if ns >= 0 {
 			var t timespec
 			t.set_nsec(ns)
-			err := pthread_cond_timedwait(&mp.cond, &mp.mutex, &t)
+			err := pthread_cond_timedwait_relative_np(&mp.cond, &mp.mutex, &t)
 			if err == _ETIMEDOUT {
 				pthread_mutex_unlock(&mp.mutex)
 				return -1

src/runtime/sys_darwin.go

@@ -287,10 +287,10 @@ func pthread_cond_wait_trampoline()
 
 //go:nosplit
 //go:cgo_unsafe_args
-func pthread_cond_timedwait(c *pthreadcond, m *pthreadmutex, t *timespec) int32 {
-	return libcCall(unsafe.Pointer(funcPC(pthread_cond_timedwait_trampoline)), unsafe.Pointer(&c))
+func pthread_cond_timedwait_relative_np(c *pthreadcond, m *pthreadmutex, t *timespec) int32 {
+	return libcCall(unsafe.Pointer(funcPC(pthread_cond_timedwait_relative_np_trampoline)), unsafe.Pointer(&c))
 }
-func pthread_cond_timedwait_trampoline()
+func pthread_cond_timedwait_relative_np_trampoline()
 
 //go:nosplit
 //go:cgo_unsafe_args
@@ -348,7 +348,7 @@ func closeonexec(fd int32) {
 //go:cgo_import_dynamic libc_pthread_mutex_unlock pthread_mutex_unlock "/usr/lib/libSystem.B.dylib"
 //go:cgo_import_dynamic libc_pthread_cond_init pthread_cond_init "/usr/lib/libSystem.B.dylib"
 //go:cgo_import_dynamic libc_pthread_cond_wait pthread_cond_wait "/usr/lib/libSystem.B.dylib"
-//go:cgo_import_dynamic libc_pthread_cond_timedwait pthread_cond_timedwait "/usr/lib/libSystem.B.dylib"
+//go:cgo_import_dynamic libc_pthread_cond_timedwait_relative_np pthread_cond_timedwait_relative_np "/usr/lib/libSystem.B.dylib"
 //go:cgo_import_dynamic libc_pthread_cond_signal pthread_cond_signal "/usr/lib/libSystem.B.dylib"
 
 // Magic incantation to get libSystem actually dynamically linked.

src/runtime/sys_darwin_386.s

@@ -595,7 +595,7 @@ TEXT runtime·pthread_cond_wait_trampoline(SB),NOSPLIT,$0
 	POPL	BP
 	RET
 
-TEXT runtime·pthread_cond_timedwait_trampoline(SB),NOSPLIT,$0
+TEXT runtime·pthread_cond_timedwait_relative_np_trampoline(SB),NOSPLIT,$0
 	PUSHL	BP
 	MOVL	SP, BP
 	SUBL	$24, SP
@@ -606,7 +606,7 @@ TEXT runtime·pthread_cond_timedwait_trampoline(SB),NOSPLIT,$0
 	MOVL	AX, 4(SP)
 	MOVL	8(CX), AX	// arg 3 timeout
 	MOVL	AX, 8(SP)
-	CALL	libc_pthread_cond_timedwait(SB)
+	CALL	libc_pthread_cond_timedwait_relative_np(SB)
 	MOVL	BP, SP
 	POPL	BP
 	RET

src/runtime/sys_darwin_amd64.s

@@ -447,13 +447,13 @@ TEXT runtime·pthread_cond_wait_trampoline(SB),NOSPLIT,$0
 	POPQ	BP
 	RET
 
-TEXT runtime·pthread_cond_timedwait_trampoline(SB),NOSPLIT,$0
+TEXT runtime·pthread_cond_timedwait_relative_np_trampoline(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP
 	MOVQ	8(DI), SI	// arg 2 mutex
 	MOVQ	16(DI), DX	// arg 3 timeout
 	MOVQ	0(DI), DI	// arg 1 cond
-	CALL	libc_pthread_cond_timedwait(SB)
+	CALL	libc_pthread_cond_timedwait_relative_np(SB)
 	POPQ	BP
 	RET
 

src/runtime/sys_darwin_arm.s

@@ -368,11 +368,11 @@ TEXT runtime·pthread_cond_wait_trampoline(SB),NOSPLIT,$0
 	BL	libc_pthread_cond_wait(SB)
 	RET
 
-TEXT runtime·pthread_cond_timedwait_trampoline(SB),NOSPLIT,$0
+TEXT runtime·pthread_cond_timedwait_relative_np_trampoline(SB),NOSPLIT,$0
 	MOVW	4(R0), R1	// arg 2 mutex
 	MOVW	8(R0), R2	// arg 3 timeout
 	MOVW	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_timedwait(SB)
+	BL	libc_pthread_cond_timedwait_relative_np(SB)
 	RET
 
 TEXT runtime·pthread_cond_signal_trampoline(SB),NOSPLIT,$0

src/runtime/sys_darwin_arm64.s

@@ -357,11 +357,11 @@ TEXT runtime·pthread_cond_wait_trampoline(SB),NOSPLIT,$0
 	BL	libc_pthread_cond_wait(SB)
 	RET
 
-TEXT runtime·pthread_cond_timedwait_trampoline(SB),NOSPLIT,$0
+TEXT runtime·pthread_cond_timedwait_relative_np_trampoline(SB),NOSPLIT,$0
 	MOVD	8(R0), R1	// arg 2 mutex
 	MOVD	16(R0), R2	// arg 3 timeout
 	MOVD	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_timedwait(SB)
+	BL	libc_pthread_cond_timedwait_relative_np(SB)
 	RET
 
 TEXT runtime·pthread_cond_signal_trampoline(SB),NOSPLIT,$0