linux-kernel - Re: [PATCH] stack and rcu interaction bug in smp_call_function

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Date:	Sat, 09 Aug 2008 02:14:18 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>, Ingo Molnar <mingo@...e.hu>,
	npiggin@...e.de, linux-kernel <linux-kernel@...r.kernel.org>,
	suresh.b.siddha@...el.com
Subject: Re: [PATCH] stack and rcu interaction bug in smp_call_function_mask()

Venki Pallipadi <venkatesh.pallipadi@...el.com> writes:
>
> One way to solve the problem is to have CPU A wait as long as there is a rcu
> read happening. But, we cannot use synchronize_rcu() here as we cannot block.
> Another way around is to always allocate call_function_data, instead
> of using CPU A's stack. Below patch does this. But, that will still have to
> handle the kmalloc failure case somehow.
>
> Any other ideas on how to solve this problem?

Perhaps it needs a custom RCU cycle? That is possible to define
with some effort.

I would feel uneasy about always allocating. GFP_ATOMIC can fail and I
expect most callers are not prepared to do handle that and in many
cases there is not even a good way. Now that there are patches
floating around even using this for the tlb flush that would be
deadly.

-Andi
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