config: fix dequeue_signal check for kernels <4.20

Before 4.20, kernel_siginfo_t was just called siginfo_t. This was
causing the kthread_dequeue_signal_3arg_task check, which uses
kernel_siginfo_t, to fail on older kernels.

In d6b8c17, we started checking for the "new" three-arg
dequeue_signal() by testing for the "old" version. Because that test is
explicitly using kernel_siginfo_t, it would fail, leading to the build
trying to use the new three-arg version, which would then not compile.

This commit fixes that by avoiding checking for the old 3-arg
dequeue_signal entirely. Instead, we check for the new one, as well as
the 4-arg form, and we use the old form as a fallback. This way, we
never have to test for it explicitly, and once we're building
HAVE_SIGINFO will make sure we get the right kernel_siginfo_t for it, so
everything works out nice.

Original-patch-by: Finix <yancw@info2soft.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes openzfs#16666

config/kernel-kthread.m4

@@ -17,14 +17,21 @@ AC_DEFUN([ZFS_AC_KERNEL_KTHREAD_COMPLETE_AND_EXIT], [
 
 AC_DEFUN([ZFS_AC_KERNEL_KTHREAD_DEQUEUE_SIGNAL], [
 	dnl #
-	dnl # 5.17 API: enum pid_type * as new 4th dequeue_signal() argument,
-	dnl # 5768d8906bc23d512b1a736c1e198aa833a6daa4 ("signal: Requeue signals in the appropriate queue")
+	dnl # prehistory:
+	dnl #     int dequeue_signal(struct task_struct *task, sigset_t *mask,
+	dnl #         siginfo_t *info)
 	dnl #
-	dnl # int dequeue_signal(struct task_struct *task, sigset_t *mask, kernel_siginfo_t *info);
-	dnl # int dequeue_signal(struct task_struct *task, sigset_t *mask, kernel_siginfo_t *info, enum pid_type *type);
+	dnl # 4.20: kernel_siginfo_t introduced, replaces siginfo_t
+	dnl #     int dequeue_signal(struct task_struct *task, sigset_t *mask,
+	dnl           kernel_siginfo_t *info)
 	dnl #
-	dnl # 6.12 API: first arg struct_task* removed
-	dnl # int dequeue_signal(sigset_t *mask, kernel_siginfo_t *info, enum pid_type *type);
+	dnl # 5.17: enum pid_type introduced as 4th arg
+	dnl #     int dequeue_signal(struct task_struct *task, sigset_t *mask,
+	dnl #         kernel_siginfo_t *info, enum pid_type *type)
+	dnl #
+	dnl # 6.12: first arg struct_task* removed
+	dnl #     int dequeue_signal(sigset_t *mask, kernel_siginfo_t *info,
+	dnl #         enum pid_type *type)
 	dnl #
 	AC_MSG_CHECKING([whether dequeue_signal() takes 4 arguments])
 	ZFS_LINUX_TEST_RESULT([kthread_dequeue_signal_4arg], [
@@ -33,11 +40,11 @@ AC_DEFUN([ZFS_AC_KERNEL_KTHREAD_DEQUEUE_SIGNAL], [
 		    [dequeue_signal() takes 4 arguments])
 	], [
 		AC_MSG_RESULT(no)
-		AC_MSG_CHECKING([whether dequeue_signal() a task argument])
-		ZFS_LINUX_TEST_RESULT([kthread_dequeue_signal_3arg_task], [
+		AC_MSG_CHECKING([whether 3-arg dequeue_signal() takes a type argument])
+		ZFS_LINUX_TEST_RESULT([kthread_dequeue_signal_3arg_type], [
 			AC_MSG_RESULT(yes)
-			AC_DEFINE(HAVE_DEQUEUE_SIGNAL_3ARG_TASK, 1,
-			    [dequeue_signal() takes a task argument])
+			AC_DEFINE(HAVE_DEQUEUE_SIGNAL_3ARG_TYPE, 1,
+			    [3-arg dequeue_signal() takes a type argument])
 		], [
 			AC_MSG_RESULT(no)
 		])
@@ -56,27 +63,27 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_KTHREAD_COMPLETE_AND_EXIT], [
 ])
 
 AC_DEFUN([ZFS_AC_KERNEL_SRC_KTHREAD_DEQUEUE_SIGNAL], [
-	ZFS_LINUX_TEST_SRC([kthread_dequeue_signal_3arg_task], [
+	ZFS_LINUX_TEST_SRC([kthread_dequeue_signal_4arg], [
 		#include <linux/sched/signal.h>
 	], [
 		struct task_struct *task = NULL;
 		sigset_t *mask = NULL;
 		kernel_siginfo_t *info = NULL;
+		enum pid_type *type = NULL;
 		int error __attribute__ ((unused));
 
-		error = dequeue_signal(task, mask, info);
+		error = dequeue_signal(task, mask, info, type);
 	])
 
-	ZFS_LINUX_TEST_SRC([kthread_dequeue_signal_4arg], [
+	ZFS_LINUX_TEST_SRC([kthread_dequeue_signal_3arg_type], [
 		#include <linux/sched/signal.h>
 	], [
-		struct task_struct *task = NULL;
 		sigset_t *mask = NULL;
 		kernel_siginfo_t *info = NULL;
 		enum pid_type *type = NULL;
 		int error __attribute__ ((unused));
 
-		error = dequeue_signal(task, mask, info, type);
+		error = dequeue_signal(mask, info, type);
 	])
 ])
 

module/os/linux/spl/spl-thread.c

@@ -171,11 +171,11 @@ issig(void)
 #if defined(HAVE_DEQUEUE_SIGNAL_4ARG)
 	enum pid_type __type;
 	if (dequeue_signal(current, &set, &__info, &__type) != 0) {
-#elif defined(HAVE_DEQUEUE_SIGNAL_3ARG_TASK)
-	if (dequeue_signal(current, &set, &__info) != 0) {
-#else
+#elif defined(HAVE_DEQUEUE_SIGNAL_3ARG_TYPE)
 	enum pid_type __type;
 	if (dequeue_signal(&set, &__info, &__type) != 0) {
+#else
+	if (dequeue_signal(current, &set, &__info) != 0) {
 #endif
 		spin_unlock_irq(&current->sighand->siglock);
 		kernel_signal_stop();