Linux 4.11 compat: remove stub for __put_task_struct

In some kernel older than 2.6.35, something in spl or zfs needed to
link against __put_task_struct() even though it should never be called
by zfs.

Which kernel that was, and what code needed to link against
__put_task_struct(), is unknown to me.

This is no longer the case.  spl/zfs build and the modules install
successfully on a RHEL6 derivative with kernel
2.6.32-642.13.1.1chaos.ch5.5.x86_64 without the stub, as well as
4.8.0-22-generic under Ubuntu and 4.11 kernel built from
github/torvalds/linux.

This came up with the linux 4.11 kernel because the prototype for
__put_task_struct() is in a new include file and so the config check
failed to detect the exported symbol.

Removing the unnecessary stub and corresponding config check.

Signed-off-by: Olaf Faaland <[email protected]>

config/spl-build.m4

@@ -33,7 +33,6 @@ AC_DEFUN([SPL_AC_CONFIG_KERNEL], [
 	SPL_AC_INODE_TRUNCATE_RANGE
 	SPL_AC_FS_STRUCT_SPINLOCK
 	SPL_AC_KUIDGID_T
-	SPL_AC_PUT_TASK_STRUCT
 	SPL_AC_KERNEL_FALLOCATE
 	SPL_AC_CONFIG_ZLIB_INFLATE
 	SPL_AC_CONFIG_ZLIB_DEFLATE
@@ -1083,25 +1082,6 @@ AC_DEFUN([SPL_AC_KUIDGID_T], [
 	])
 ])
 
-dnl #
-dnl # 2.6.39 API change,
-dnl # __put_task_struct() was exported by the mainline kernel.
-dnl #
-AC_DEFUN([SPL_AC_PUT_TASK_STRUCT],
-	[AC_MSG_CHECKING([whether __put_task_struct() is available])
-	SPL_LINUX_TRY_COMPILE_SYMBOL([
-		#include <linux/sched.h>
-	], [
-		__put_task_struct(NULL);
-	], [__put_task_struct], [], [
-		AC_MSG_RESULT(yes)
-		AC_DEFINE(HAVE_PUT_TASK_STRUCT, 1,
-		          [__put_task_struct() is available])
-	], [
-		AC_MSG_RESULT(no)
-	])
-])
-
 dnl #
 dnl # 2.6.35 API change,
 dnl # Unused 'struct dentry *' removed from vfs_fsync() prototype.

module/spl/spl-generic.c

@@ -459,22 +459,6 @@ ddi_copyout(const void *from, void *to, size_t len, int flags)
 }
 EXPORT_SYMBOL(ddi_copyout);
 
-#ifndef HAVE_PUT_TASK_STRUCT
-/*
- * This is only a stub function which should never be used.  The SPL should
- * never be putting away the last reference on a task structure so this will
- * not be called.  However, we still need to define it so the module does not
- * have undefined symbol at load time.  That all said if this impossible
- * thing does somehow happen PANIC immediately so we know about it.
- */
-void
-__put_task_struct(struct task_struct *t)
-{
-	PANIC("Unexpectly put last reference on task %d\n", (int)t->pid);
-}
-EXPORT_SYMBOL(__put_task_struct);
-#endif /* HAVE_PUT_TASK_STRUCT */
-
 /*
  * Read the unique system identifier from the /etc/hostid file.
  *