linux-kernel - Re: [PATCH 1/3] coredump: flush the fpu exit state for proper multi-threaded core dump

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Message-ID: <20120509210551.GB334@redhat.com>
Date:	Wed, 9 May 2012 23:05:51 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	torvalds@...ux-foundation.org, hpa@...or.com, mingo@...e.hu,
	linux-kernel@...r.kernel.org, suresh@...stanetworks.com
Subject: Re: [PATCH 1/3] coredump: flush the fpu exit state for proper
	multi-threaded core dump

On 05/08, Suresh Siddha wrote:
>
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -656,6 +656,11 @@ static void exit_mm(struct task_struct * tsk)
>  		struct core_thread self;
>  		up_read(&mm->mmap_sem);
>
> +		/*
> +		 * Flush the live extended register state to memory.
> +		 */
> +		prepare_to_copy(tsk);

This doesn't look very nice imho, but I guess you understand this...

Perhaps we need an arch-dependent helper which saves the FPU regs
if needed.

I can be easily wrong, but I did the quick grep and I am not sure
we can rely on prepare_to_copy(). For example, it is a nop in
arch/sh/include/asm/processor_64.h. But at the same time it has
save_fpu().

OTOH, I am not sure it is safe to use prepare_to_copy() in exit_mm(),
at least in theory. God knows what it can do...

But again, I do not think I can comment this change. Perhaps this
is the right step anyway.

Oleg.

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