linux-kernel - Re: [PATCH 0/3] minor cleanups to EFLAGS initialisation in ret_from

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Date:	Tue, 26 Jul 2011 16:46:34 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 0/3] minor cleanups to EFLAGS initialisation in
 ret_from_fork

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.
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