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

flush_thread() -> drop_init_fpu() is suboptimal and confusing. It does
drop_fpu() or restore_init_xstate() depending on !use_eager_fpu(). But
flush_thread() too checks eagerfpu right after that, and if it is true
then restore_init_xstate() just burns CPU for no reason. We are going to
load init_xstate_buf again after we set used_math()/user_has_fpu(), until
then the FPU state can't survive after switch_to().

Remove it, and change the "if (!use_eager_fpu())" to call drop_fpu().
While at it, clean up the tsk/current usage.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.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>
Link: http://lkml.kernel.org/r/20150313173030.GA31217@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 6 insertions, 9 deletions

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 6b058296a456..1d2ebadba7ac 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -132,17 +132,14 @@ void flush_thread(void)
         flush_ptrace_hw_breakpoint(tsk);
         memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));
 
-        drop_init_fpu(tsk);
-        /*
-         * Free the FPU state for non xsave platforms. They get reallocated
-         * lazily at the first use.
-         */
-        if (!use_eager_fpu())
+        if (!use_eager_fpu()) {
+                /* FPU state will be reallocated lazily at the first use. */
+                drop_fpu(tsk);
                 free_thread_xstate(tsk);
-        else if (!used_math()) {
+        } else if (!used_math()) {
                 /* kthread execs. TODO: cleanup this horror. */
-                if (WARN_ON(init_fpu(current)))
-                        force_sig(SIGKILL, current);
+                if (WARN_ON(init_fpu(tsk)))
+                        force_sig(SIGKILL, tsk);
                 user_fpu_begin();
                 restore_init_xstate();
         }