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

H. Peter Anvin
2011-07-25T21:10:20Z

On 07/25/2011 11:20 AM, Cyrill Gorcunov wrote:
> 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...
>

This definitely doesn't look "obviously safe" to me. However, does
anyone see a problem with unconditionally leaving IF disabled even on 32
bits (I haven't traced all the paths yet), i.e. doing the *opposite* of
Ian's patch #2?

-hpa