x86/fpu: Always restore_xinit_state() when use_eager_cpu()

The following commit:

  9ddf87659634 ("x86/fpu: Don't abuse drop_init_fpu() in flush_thread()")

removed drop_init_fpu() usage from flush_thread(). This seems to break
things for me - the Go 1.4 test suite fails all over the place with
floating point comparision errors (offending commit found through
bisection).

The functional change was that flush_thread() after this commit
only calls restore_init_xstate() when both use_eager_fpu() and
!used_math() are true. drop_init_fpu() (now fpu_reset_state()) calls
restore_init_xstate() regardless of whether current used_math() - apply
the same logic here.

Switch used_math() -> tsk_used_math(tsk) to consistently use the grabbed
tsk instead of current, like in the rest of flush_thread().

Tested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Bobby Powers <bobbypowers@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pekka Riikonen <priikone@iki.fi>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 9ddf8765 ("x86/fpu: Don't abuse drop_init_fpu() in flush_thread()")
Link: http://lkml.kernel.org/r/1430147441-9820-1-git-send-email-bobbypowers@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index bfc99b3b6522078ba2aea707b24d6aa513e1bf7d..6e338e3b1dc04cc69ab41c012fe5671cc25321cd 100644 (file)
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -156,11 +156,13 @@ void flush_thread(void)
                /* FPU state will be reallocated lazily at the first use. */
                drop_fpu(tsk);
                free_thread_xstate(tsk);
-       } else if (!used_math()) {
-               /* kthread execs. TODO: cleanup this horror. */
-               if (WARN_ON(init_fpu(tsk)))
-                       force_sig(SIGKILL, tsk);
-               user_fpu_begin();
+       } else {
+               if (!tsk_used_math(tsk)) {
+                       /* kthread execs. TODO: cleanup this horror. */
+                       if (WARN_ON(init_fpu(tsk)))
+                               force_sig(SIGKILL, tsk);
+                       user_fpu_begin();
+               }
                restore_init_xstate();
        }
 }