linux-kernel - [PATCH] x86/fpu: drop_fpu() should not assume that tsk == current

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Message-ID: <20150309171041.GB11388@redhat.com>
Date:	Mon, 9 Mar 2015 18:10:41 +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] x86/fpu: drop_fpu() should not assume that tsk == current

drop_fpu() does clear_used_math() and usually this is correct because
tsk == current. However switch_fpu_finish()->restore_fpu_checking() is
called before it updates the "current_task" variable. If it fails, we
will wrongly clear the PF_USED_MATH flag of the previous task.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Cc: <stable@...r.kernel.org>
---
 arch/x86/include/asm/fpu-internal.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index 99a2067..81c86fa 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -336,7 +336,7 @@ static inline void drop_fpu(struct task_struct *tsk)
 	preempt_disable();
 	tsk->thread.fpu_counter = 0;
 	__drop_fpu(tsk);
-	clear_used_math();
+	clear_stopped_child_used_math(tsk);
 	preempt_enable();
 }
 
-- 
1.5.5.1


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