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Subject[PATCH 3.16.y-ckt 050/144] x86/fpu: Drop_fpu() should not assume that tsk equals current
Date
3.16.7-ckt10 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@redhat.com>

commit f4c3686386393c120710dd34df2a74183ab805fd upstream.

drop_fpu() does clear_used_math() and usually this is correct
because tsk == current.

However switch_fpu_finish()->restore_fpu_checking() is called before
__switch_to() updates the "current_task" variable. If it fails,
we will wrongly clear the PF_USED_MATH flag of the previous task.

So use clear_stopped_child_used_math() instead.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pekka Riikonen <priikone@iki.fi>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150309171041.GB11388@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
arch/x86/include/asm/fpu-internal.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index 115e3689cd53..0eb93c52e8d4 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -368,7 +368,7 @@ 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();
}


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