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From: Borislav Petkov <bp@alien8.de>
To: riel@redhat.com, oleg@redhat.com
Cc: dave.hansen@linux.intel.com, sbsiddha@gmail.com,
	luto@amacapital.net, tglx@linutronix.de, mingo@kernel.org,
	hpa@zytor.com, fenghua.yu@intel.com, x86@kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/8] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if use_eager_fpu()
Date: Mon, 16 Feb 2015 21:25:40 +0100	[thread overview]
Message-ID: <20150216202540.GM4458@pd.tnic> (raw)
In-Reply-To: <1423252925-14451-3-git-send-email-riel@redhat.com>

On Fri, Feb 06, 2015 at 03:01:59PM -0500, riel@redhat.com wrote:
> From: Oleg Nesterov <oleg@redhat.com>
> 
> 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.

I can follow so far.

> - 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.

Well, my primitive understanding would say no, not safely, for the
simple reason that we have only one XSAVE state area per thread.
However, __kernel_fpu_begin() is called with preemption disabled so ...
I guess I'm still not seeing the race.

Btw, what is kernel_fpu_disable()? Can't find it here.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--

  reply	other threads:[~2015-02-16 20:26 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-06 20:01 [PATCH 0/8] x86,fpu: various small FPU cleanups and optimizations riel
2015-02-06 20:01 ` [PATCH 1/8] x86, fpu: unlazy_fpu: don't reset thread.fpu_counter riel
2015-02-16 17:04   ` Borislav Petkov
2015-02-16 17:58     ` Rik van Riel
2015-02-16 18:14       ` Oleg Nesterov
2015-02-16 18:16         ` Borislav Petkov
2015-02-19 11:32   ` [tip:x86/fpu] x86/fpu: Don't " tip-bot for Oleg Nesterov
2015-02-06 20:01 ` [PATCH 2/8] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if use_eager_fpu() riel
2015-02-16 20:25   ` Borislav Petkov [this message]
2015-02-17 10:47     ` Oleg Nesterov
2015-02-17 12:09       ` Borislav Petkov
2015-02-19 11:32   ` [tip:x86/fpu] x86/fpu: Don't " tip-bot for Oleg Nesterov
2015-02-06 20:02 ` [PATCH 3/8] x86, fpu: kill save_init_fpu(), change math_error() to use unlazy_fpu() riel
2015-02-16 21:09   ` Borislav Petkov
2015-02-16 21:30     ` Rik van Riel
2015-02-17 10:58       ` Oleg Nesterov
2015-02-19 11:32   ` [tip:x86/fpu] x86/fpu: Change math_error() to use unlazy_fpu(), kill (now) unused save_init_fpu() tip-bot for Oleg Nesterov
2015-02-06 20:02 ` [PATCH 4/8] x86,fpu: move lazy restore functions up a few lines riel
2015-02-19 11:33   ` [tip:x86/fpu] x86/fpu: Move " tip-bot for Rik van Riel
2015-02-06 20:02 ` [PATCH 5/8] x86,fpu: introduce task_disable_lazy_fpu_restore helper riel
2015-02-19 11:33   ` [tip:x86/fpu] x86/fpu: Introduce task_disable_lazy_fpu_restore() helper tip-bot for Rik van Riel
2015-02-06 20:02 ` [PATCH 6/8] x86,fpu: use an explicit if/else in switch_fpu_prepare riel
2015-02-17  8:44   ` Borislav Petkov
2015-02-19 11:33   ` [tip:x86/fpu] x86/fpu: Use an explicit if/ else in switch_fpu_prepare() tip-bot for Rik van Riel
2015-02-06 20:02 ` [PATCH 7/8] x86,fpu: use disable_task_lazy_fpu_restore helper riel
2015-02-17  9:00   ` Borislav Petkov
2015-02-17 11:04     ` Oleg Nesterov
2015-02-17 12:11       ` Borislav Petkov
2015-02-19 11:34   ` [tip:x86/fpu] x86/fpu: Use task_disable_lazy_fpu_restore() helper tip-bot for Rik van Riel
2015-02-06 20:02 ` [PATCH 8/8] x86,fpu: also check fpu_lazy_restore when use_eager_fpu riel
2015-02-19 11:34   ` [tip:x86/fpu] x86/fpu: Also check fpu_lazy_restore() when use_eager_fpu() tip-bot for Rik van Riel
2015-02-16 15:26 ` [PATCH 0/8] x86,fpu: various small FPU cleanups and optimizations Rik van Riel
2015-02-16 16:00   ` Borislav Petkov

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