From: "H. Peter Anvin" <hpa@zytor.com>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>,
linux-kernel@vger.kernel.org, Pekka Enberg <penberg@kernel.org>,
Andi Kleen <ak@linux.intel.com>, Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH 0/3] minor cleanups to EFLAGS initialisation in ret_from_fork
Date: Mon, 25 Jul 2011 14:10:02 -0700 [thread overview]
Message-ID: <4E2DDBAA.60200@zytor.com> (raw)
In-Reply-To: <20110725182049.GD27137@sun>
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
next prev parent reply other threads:[~2011-07-25 21:10 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-25 9:58 [PATCH 0/3] minor cleanups to EFLAGS initialisation in ret_from_fork Ian Campbell
2011-07-25 10:03 ` [PATCH 1/3] x86: drop unnecessary kernel_eflags variable from 64 bit Ian Campbell
2011-07-25 10:03 ` [PATCH 2/3] x86: make 64 bit ret_from_fork a little more similar to 32 bit Ian Campbell
2011-07-25 10:03 ` [PATCH 3/3] x86: ret_from_fork: use symbolic contants for bits in EFLAGS Ian Campbell
2011-07-25 10:19 ` [PATCH 0/3] minor cleanups to EFLAGS initialisation in ret_from_fork Cyrill Gorcunov
2011-07-25 18:20 ` Cyrill Gorcunov
2011-07-25 21:10 ` H. Peter Anvin [this message]
2011-07-25 21:47 ` Cyrill Gorcunov
2011-07-26 14:46 ` Peter Zijlstra
2011-07-26 15:51 ` Ian Campbell
2011-08-10 15:27 ` Ian Campbell
2011-08-10 15:35 ` Peter Zijlstra
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