linux-kernel - [PATCH 0/4] x86/fpu: avoid math_state

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Message-ID: <20150311173346.GB5032@redhat.com>
Date:	Wed, 11 Mar 2015 18:33:46 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Dave Hansen <dave.hansen@...el.com>, Borislav Petkov <bp@...e.de>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: [PATCH 0/4] x86/fpu: avoid math_state_restore() on kthread exec

Hello.

This should "fix" the kernel crash observed by Dave.

But let me repeat once again, the problem is that fpu_finit() is buggy
on Dave's machine. This series should "hide" this problem, we need to
fix it later anyway.

I was going to do these changes anyway, math_state_restore() was only
used because we did not have the necessary helpers. I was going to start
with init_fpu() cleanups, but since math_state_restore() makes this
fpu_finit() bug more visible lets remove it first.

Note that init_fpu() + user_fpu_begin() is racy, used_math() is already
set so __switch_to() in between can do restore_fpu_checking() too and
trigger the same GPF. But this is fine (to some degree), the task won't
be killed. And this is just another proof that init_fpu() should not
set used_math() and it and its users need more cleanups.

More to come tomorrow.

Oleg.

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