Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

Peter Zijlstra
2019-02-14T19:34:46Z

On Thu, Feb 14, 2019 at 11:18:01AM -0500, Brian Gerst wrote:

> > --- a/arch/x86/include/asm/switch_to.h
> > +++ b/arch/x86/include/asm/switch_to.h
> > @@ -40,6 +40,7 @@ asmlinkage void ret_from_fork(void);
> > * order of the fields must match the code in __switch_to_asm().
> > */
> > struct inactive_task_frame {
> > + unsigned long flags;
> > #ifdef CONFIG_X86_64
> > unsigned long r15;
> > unsigned long r14;
>
> flags should be initialized in copy_thread_tls(). I think the new
> stack is zeroed already, but it would be better to explicitly set it.

Ah indeed. I somehow misread that code and thought we got initialized
with a copy of current.

Something like the below, right?

---

--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -127,6 +127,7 @@ int copy_thread_tls(unsigned long clone_
struct task_struct *tsk;
int err;

+ frame->flags = 0;
frame->bp = 0;
frame->ret_addr = (unsigned long) ret_from_fork;
p->thread.sp = (unsigned long) fork_frame;
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -392,6 +392,7 @@ int copy_thread_tls(unsigned long clone_
childregs = task_pt_regs(p);
fork_frame = container_of(childregs, struct fork_frame, regs);
frame = &fork_frame->frame;
+ frame->flags = 0;
frame->bp = 0;
frame->ret_addr = (unsigned long) ret_from_fork;
p->thread.sp = (unsigned long) fork_frame;