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

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Date:	Mon, 25 Jul 2011 14:10:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	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 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

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