[PATCH 2/3] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if use_eager_fpu()

[Oleg Nesterov <oleg@redhat.com>]

unlazy_fpu()->__thread_fpu_end() doesn't look right if use_eager_fpu().
Unconditional __thread_fpu_end() is only correct if we know that this
thread can't return to user-mode and use FPU.

Fortunately it has only 2 callers. fpu_copy() checks use_eager_fpu(),
and init_fpu(current) can be only called by the coredumping thread via
regset->get(). But it is exported to modules, and imo this should be
fixed anyway.

And if we check use_eager_fpu() we can use __save_fpu() like fpu_copy()
and save_init_fpu() do.

- It seems that even !use_eager_fpu() case doesn't need the unconditional
  __thread_fpu_end(), we only need it if __save_init_fpu() returns 0.

- It is still not clear to me if __save_init_fpu() can safely nest with
  another save + restore from __kernel_fpu_begin(). If not, we can use
  kernel_fpu_disable() to fix the race.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/x86/kernel/i387.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index c3b92c0..8e070a6 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -120,8 +120,12 @@ void unlazy_fpu(struct task_struct *tsk)
 {
 	preempt_disable();
 	if (__thread_has_fpu(tsk)) {
-		__save_init_fpu(tsk);
-		__thread_fpu_end(tsk);
+		if (use_eager_fpu()) {
+			__save_fpu(tsk);
+		} else {
+			__save_init_fpu(tsk);
+			__thread_fpu_end(tsk);
+		}
 	}
 	preempt_enable();
 }
-- 
1.5.5.1