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

Ian Campbell
2011-07-26T15:51:39Z

On Tue, 2011-07-26 at 16:46 +0200, Peter Zijlstra wrote:
> On Tue, 2011-07-26 at 01:47 +0400, Cyrill Gorcunov wrote:
> > > > 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?
>
> Right, enabling IRQs there isn't cool, currently there's still
> __ARCH_WANT_INTERRUPTS_ON_CTXSW but we're working hard on getting rid of
> that nightmare.
>
> There's a number of very subtle things that can go wrong when you enable
> interrupts over the context switch.
>
> Leaving IRQs disabled should be the right thing, on x86 we should
> _never_ have interrupts enabled there.

Thanks Peter. I'll flip the (not any longer) "arbitrary" decision in my
#2 patch and repost when I return from debconf/vacation.

Ian.

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