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Subject[PATCH v2 4/4] 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>
---
arch/x86/kernel/process.c | 15 ++++++---------
1 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index c396de2..c236306 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -131,17 +131,14 @@ 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();
}
--
1.5.5.1



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