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

[Peter Zijlstra]

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;