[PATCH 3/3] x86, fpu: don't abuse FPU in kernel threads if use_eager_fpu()

Oleg Nesterov
2015-01-19T18:53:22Z

Afaics there is no reason why kernel threads should have FPU context
even if use_eager_fpu() == T. Now that interrupted_kernel_fpu_idle()
does not check __thread_has_fpu() we can remove the init_fpu() code
from eager_fpu_init() and change flush_thread() called by do_execve()
to initialize FPU.

Note: of course, the change in flush_thread() is horrible and must be
cleanuped. We need the new helper, and flush_thread() should return the
error if init_fpu() fails.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
arch/x86/kernel/process.c | 7 +++++++
arch/x86/kernel/xsave.c | 13 +------------
2 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index e127dda..dd9a069 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -130,6 +130,7 @@ 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
@@ -137,6 +138,12 @@ void flush_thread(void)
*/
if (!use_eager_fpu())
free_thread_xstate(tsk);
+ else if (!used_math()) {
+ /* kthread execs. TODO: cleanup this horror. */
+ if (WARN_ON(init_fpu(current)))
+ force_sig(SIGKILL, current);
+ math_state_restore();
+ }
}

static void hard_disable_TSC(void)
diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index 0de1fae..de9dcf8 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -688,7 +688,7 @@ void eager_fpu_init(void)
{
static __refdata void (*boot_func)(void) = eager_fpu_init_bp;

- clear_used_math();
+ WARN_ON(used_math());
current_thread_info()->status = 0;

if (eagerfpu == ENABLE)
@@ -703,17 +703,6 @@ void eager_fpu_init(void)
boot_func();
boot_func = NULL;
}
-
- /*
- * This is same as math_state_restore(). But use_xsave() is
- * not yet patched to use math_state_restore().
- */
- init_fpu(current);
- __thread_fpu_begin(current);
- if (cpu_has_xsave)
- xrstor_state(init_xstate_buf, -1);
- else
- fxrstor_checking(&init_xstate_buf->i387);
}

/*
--
1.5.5.1