[PATCH 4/7] x86/fpu: Split the state handling in fpu__drop()

[Ingo Molnar]

Prepare fpu__drop() to use fpu->fpregs_active.

There are two distinct usecases for fpu__drop() in this context:
exit_thread() when called for 'current' in exit(), and when called
for another task in fork().

This patch does not change behavior, it only adds a couple of
debug checks and structures the code to make the ->fpregs_active
change more obviously correct.

All the complications will be removed later on.

Cc: Andy Lutomirski <l...@kernel.org>
Cc: Borislav Petkov <b...@alien8.de>
Cc: Dave Hansen <dave.han...@linux.intel.com>
Cc: Fenghua Yu <fenghua...@intel.com>
Cc: H. Peter Anvin <h...@zytor.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Oleg Nesterov <o...@redhat.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Rik van Riel <r...@redhat.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Yu-cheng Yu <yu-cheng...@intel.com>
Signed-off-by: Ingo Molnar <mi...@kernel.org>
---
 arch/x86/kernel/fpu/core.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 7e1c77ac90e8..2973afdb39f1 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -414,12 +414,19 @@ void fpu__drop(struct fpu *fpu)
 {
        preempt_disable();
 
-       if (fpu->fpregs_active) {
-               /* Ignore delayed exceptions from user space */
-               asm volatile("1: fwait\n"
-                            "2:\n"
-                            _ASM_EXTABLE(1b, 2b));
-               fpregs_deactivate(fpu);
+       if (fpu == &current->thread.fpu) {
+               WARN_ON_FPU(fpu->fpstate_active != fpu->fpregs_active);
+
+               if (fpu->fpregs_active) {
+                       /* Ignore delayed exceptions from user space */
+                       asm volatile("1: fwait\n"
+                                    "2:\n"
+                                    _ASM_EXTABLE(1b, 2b));
+                       if (fpu->fpregs_active)
+                               fpregs_deactivate(fpu);
+               }
+       } else {
+               WARN_ON_FPU(fpu->fpregs_active);
        }
 
        fpu->fpstate_active = 0;
-- 
2.7.4