linux-kernel - Re: [PATCH v9 05/12] x86, mpx: on-demand kernel allocation of bounds tables

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Message-ID: <alpine.DEB.2.11.1410241257300.5308@nanos>
Date:	Fri, 24 Oct 2014 14:08:11 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Qiaowei Ren <qiaowei.ren@...el.com>
cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-mips@...ux-mips.org
Subject: Re: [PATCH v9 05/12] x86, mpx: on-demand kernel allocation of bounds
 tables

On Sun, 12 Oct 2014, Qiaowei Ren wrote:
> +	/*
> +	 * Go poke the address of the new bounds table in to the
> +	 * bounds directory entry out in userspace memory.  Note:
> +	 * we may race with another CPU instantiating the same table.
> +	 * In that case the cmpxchg will see an unexpected
> +	 * 'actual_old_val'.
> +	 */
> +	ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry,
> +					   expected_old_val, bt_addr);

This is fully preemptible non-atomic context, right?

So this wants a proper comment, why using
user_atomic_cmpxchg_inatomic() is the right thing to do here.

Thanks,

	tglx
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