x86, fpu: Introduce per-cpu in_kernel_fpu state

interrupted_kernel_fpu_idle() tries to detect if kernel_fpu_begin()
is safe or not. In particular it should obviously deny the nested
kernel_fpu_begin() and this logic looks very confusing.

If use_eager_fpu() == T we rely on a) __thread_has_fpu() check in
interrupted_kernel_fpu_idle(), and b) on the fact that _begin() does
__thread_clear_has_fpu().

Otherwise we demand that the interrupted task has no FPU if it is in
kernel mode, this works because __kernel_fpu_begin() does clts() and
interrupted_kernel_fpu_idle() checks X86_CR0_TS.

Add the per-cpu "bool in_kernel_fpu" variable, and change this code
to check/set/clear it. This allows to do more cleanups and fixes, see
the next changes.

The patch also moves WARN_ON_ONCE() under preempt_disable() just to
make this_cpu_read() look better, this is not really needed. And in
fact I think we should move it into __kernel_fpu_begin().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: matt.fleming@intel.com
Cc: bp@suse.de
Cc: pbonzini@redhat.com
Cc: luto@amacapital.net
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Link: http://lkml.kernel.org/r/20150115191943.GB27332@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/include/asm/i387.h

@@ -40,8 +40,8 @@ extern void __kernel_fpu_end(void);
 
 static inline void kernel_fpu_begin(void)
 {
-	WARN_ON_ONCE(!irq_fpu_usable());
 	preempt_disable();
+	WARN_ON_ONCE(!irq_fpu_usable());
 	__kernel_fpu_begin();
 }
 

arch/x86/kernel/i387.c

@@ -19,6 +19,8 @@
 #include <asm/fpu-internal.h>
 #include <asm/user.h>
 
+static DEFINE_PER_CPU(bool, in_kernel_fpu);
+
 /*
  * Were we in an interrupt that interrupted kernel mode?
  *
@@ -33,6 +35,9 @@
  */
 static inline bool interrupted_kernel_fpu_idle(void)
 {
+	if (this_cpu_read(in_kernel_fpu))
+		return false;
+
 	if (use_eager_fpu())
 		return __thread_has_fpu(current);
 
@@ -73,6 +78,8 @@ void __kernel_fpu_begin(void)
 {
 	struct task_struct *me = current;
 
+	this_cpu_write(in_kernel_fpu, true);
+
 	if (__thread_has_fpu(me)) {
 		__thread_clear_has_fpu(me);
 		__save_init_fpu(me);
@@ -99,6 +106,8 @@ void __kernel_fpu_end(void)
 	} else {
 		stts();
 	}
+
+	this_cpu_write(in_kernel_fpu, false);
 }
 EXPORT_SYMBOL(__kernel_fpu_end);