linux-kernel - Re: [PATCH 1/3] x86, fpu: __kernel_fpu_begin() should clear fpu_owner_task even if use_eager

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Message-ID: <54BE630E.4000901@redhat.com>
Date:	Tue, 20 Jan 2015 09:15:42 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Suresh Siddha <sbsiddha@...il.com>
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	matt.fleming@...el.com, bp@...e.de, pbonzini@...hat.com,
	tglx@...utronix.de, luto@...capital.net
Subject: Re: [PATCH 1/3] x86, fpu: __kernel_fpu_begin() should clear fpu_owner_task
 even if use_eager_fpu()

On 01/19/2015 01:51 PM, Oleg Nesterov wrote:
> __kernel_fpu_begin() does nothing if !__thread_has_fpu() && use_eager_fpu(),
> perhaps it assumes that this case is simply impossible. This is certainly
> not possible if in_interrupt() == T; interrupted_user_mode() should have
> FPU, and interrupted_kernel_fpu_idle() should fail if !__thread_has_fpu().
>
> However, even if use_eager_fpu() == T a task can do drop_fpu(), then switch
> to another thread which becomes fpu_owner_task, then resume and call some
> function which does kernel_fpu_begin(). Say, an exiting task does a lot of
> things after exit_thread(), it is not safe to assume that it can't use FPU
> in these paths.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

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

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