x86/fpu: Introduce task_disable_lazy_fpu_restore() helper

Currently there are a few magic assignments sprinkled through the
code that disable lazy FPU state restoring, some more effective than
others, and all equally mystifying.

It would be easier to have a helper to explicitly disable lazy
FPU state restoring for a task.

Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Link: http://lkml.kernel.org/r/1423252925-14451-6-git-send-email-riel@redhat.com
Signed-off-by: Borislav Petkov <bp@suse.de>

arch/x86/include/asm/fpu-internal.h

@@ -79,6 +79,16 @@ static inline void __cpu_disable_lazy_restore(unsigned int cpu)
 	per_cpu(fpu_owner_task, cpu) = NULL;
 }
 
+/*
+ * Used to indicate that the FPU state in memory is newer than the FPU
+ * state in registers, and the FPU state should be reloaded next time the
+ * task is run. Only safe on the current task, or non-running tasks.
+ */
+static inline void task_disable_lazy_fpu_restore(struct task_struct *tsk)
+{
+	tsk->thread.fpu.last_cpu = ~0;
+}
+
 static inline int fpu_lazy_restore(struct task_struct *new, unsigned int cpu)
 {
 	return new == this_cpu_read_stable(fpu_owner_task) &&