[PATCH 2/4] x86, fpu: don't abuse ->has_fpu in __kernel_fpu_begin/end

Oleg Nesterov
2014-08-29T18:19:07Z

Now that we have in_kernel_fpu we can remove __thread_clear_has_fpu()
in __kernel_fpu_begin(). And this allow to replace the asymmetrical
and nontrivial use_eager_fpu+tsk_used_math check in kernel_fpu_end()
with the same __thread_has_fpu().

The logic becomes really simple afaics: if _begin() does save() then
_end() needs restore(), and this is controlled by __thread_has_fpu().

Also, with this patch __kernel_fpu_end() does restore_fpu_checking()
and WARNs if it fails instead of math_state_restore(). I think this
looks better because we no longer need __thread_fpu_begin(), and it
would be better to report the failure in this case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
arch/x86/kernel/i387.c | 19 ++++++-------------
1 files changed, 6 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 8fb8868..19dd36d 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -82,9 +82,7 @@ void __kernel_fpu_begin(void)

/* FIXME: race with math_state_restore()-like code */
if (__thread_has_fpu(me)) {
- __thread_clear_has_fpu(me);
__save_init_fpu(me);
- /* We do 'stts()' in __kernel_fpu_end() */
} else if (!use_eager_fpu()) {
this_cpu_write(fpu_owner_task, NULL);
clts();
@@ -94,17 +92,12 @@ EXPORT_SYMBOL(__kernel_fpu_begin);

void __kernel_fpu_end(void)
{
- if (use_eager_fpu()) {
- /*
- * For eager fpu, most the time, tsk_used_math() is true.
- * Restore the user math as we are done with the kernel usage.
- * At few instances during thread exit, signal handling etc,
- * tsk_used_math() is false. Those few places will take proper
- * actions, so we don't need to restore the math here.
- */
- if (likely(tsk_used_math(current)))
- math_state_restore();
- } else {
+ struct task_struct *me = current;
+
+ if (__thread_has_fpu(me)) {
+ if (WARN_ON(restore_fpu_checking(me)))
+ drop_init_fpu(me);
+ } else if (!use_eager_fpu()) {
stts();
}

--
1.5.5.1