linux-kernel - Re: [PATCH 2/3] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if use_eager

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Message-ID: <54CAAC01.7000000@redhat.com>
Date:	Thu, 29 Jan 2015 16:54:09 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>
CC:	Suresh Siddha <sbsiddha@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] x86, fpu: unlazy_fpu: don't do __thread_fpu_end()
 if use_eager_fpu()

On 01/29/2015 04:08 PM, Oleg Nesterov wrote:
> unlazy_fpu()->__thread_fpu_end() doesn't look right if use_eager_fpu().
> Unconditional __thread_fpu_end() is only correct if we know that this
> thread can't return to user-mode and use FPU.
> 
> Fortunately it has only 2 callers. fpu_copy() checks use_eager_fpu(),
> and init_fpu(current) can be only called by the coredumping thread via
> regset->get(). But it is exported to modules, and imo this should be
> fixed anyway.
> 
> And if we check use_eager_fpu() we can use __save_fpu() like fpu_copy()
> and save_init_fpu() do.
> 
> - It seems that even !use_eager_fpu() case doesn't need the unconditional
>   __thread_fpu_end(), we only need it if __save_init_fpu() returns 0.
> 
> - It is still not clear to me if __save_init_fpu() can safely nest with
>   another save + restore from __kernel_fpu_begin(). If not, we can use
>   kernel_fpu_disable() to fix the race.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Reviewed-by: Rik van Riel <riel@...hat.com>

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