Re: [PATCH 0/3] minor cleanups to EFLAGS initialisation in ret_from_fork

Cyrill Gorcunov
2011-07-25T18:21:00Z

On Mon, Jul 25, 2011 at 02:19:02PM +0400, Cyrill Gorcunov wrote:
> On Mon, Jul 25, 2011 at 10:58:03AM +0100, Ian Campbell wrote:
> > The following series removes the use of a global kernel_eflags variable
> > from the x86_64 ret_from_fork path and (very slightly) merges the 32 and
> > 64 bit version of that code path.
> >
> > kernel_eflags could be made a __read_mostly but actually there is no
> > reason to prefer the value at cpu_init() time to a compile time constant
> > value for the initial eflags after a fork.
> >
> > Ian.
> >
>
> Thanks, Ian! I think noone against this simplification, Peter, Andi?
>
> Cyrill

Ian, I've missed in first place that you've opened IRQs window _before_
schedule_tail() call, ie it's not 1:1 code mapping as it was before.

Note kernel_eflags has IF clear and what we have: the ret_from_fork on
x86-64 happens _only_ inside context_switch call, ie

schedule (sched.c)
...
raw_spin_lock_irq
...
context_switch
switch_to
"jnz ret_from_fork\n\t"
pushq_cfi kernel_eflags(%rip)
popfq_cfi # reset kernel eflags

---> irqs are still disabled

call schedule_tail # rdi: 'prev' task parameter
finish_lock_switch
raw_spin_unlock_irq

I bet raw_spin_lock_irq at the beginning of the schedule() is set
for a reason and such change is not safe. Though I may be missing
something again...

Cyrill