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

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Date:	Mon, 25 Jul 2011 22:20:50 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>,
	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, 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 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
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