sched/core, x86: Make struct thread_info arch specific again

The following commit:

  c65eacbe29 ("sched/core: Allow putting thread_info into task_struct")

... made 'struct thread_info' a generic struct with only a
single ::flags member, if CONFIG_THREAD_INFO_IN_TASK_STRUCT=y is
selected.

This change however seems to be quite x86 centric, since at least the
generic preemption code (asm-generic/preempt.h) assumes that struct
thread_info also has a preempt_count member, which apparently was not
true for x86.

We could add a bit more #ifdefs to solve this problem too, but it seems
to be much simpler to make struct thread_info arch specific
again. This also makes the conversion to THREAD_INFO_IN_TASK_STRUCT a
bit easier for architectures that have a couple of arch specific stuff
in their thread_info definition.

The arch specific stuff _could_ be moved to thread_struct. However
keeping them in thread_info makes it easier: accessing thread_info
members is simple, since it is at the beginning of the task_struct,
while the thread_struct is at the end. At least on s390 the offsets
needed to access members of the thread_struct (with task_struct as
base) are too large for various asm instructions.  This is not a
problem when keeping these members within thread_info.

Signed-off-by: Heiko Carstens <[email protected]>
Signed-off-by: Mark Rutland <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/include/asm/thread_info.h

@@ -52,6 +52,15 @@ struct task_struct;
 #include <asm/cpufeature.h>
 #include <linux/atomic.h>
 
+struct thread_info {
+	unsigned long		flags;		/* low level flags */
+};
+
+#define INIT_THREAD_INFO(tsk)			\
+{						\
+	.flags		= 0,			\
+}
+
 #define init_stack		(init_thread_union.stack)
 
 #else /* !__ASSEMBLY__ */