linux-kernel - Re: [RFC PATCH 01/15] cmpxchg_local() is not signed-value safe, so fix generic atomics

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Message-ID: <3591761.rzLMs6OGtT@wuerfel>
Date:	Wed, 18 May 2016 17:29:49 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	David Howells <dhowells@...hat.com>
Cc:	linux-arch@...r.kernel.org, x86@...nel.org, will.deacon@....com,
	linux-kernel@...r.kernel.org, ramana.radhakrishnan@....com,
	paulmck@...ux.vnet.ibm.com, dwmw2@...radead.org
Subject: Re: [RFC PATCH 01/15] cmpxchg_local() is not signed-value safe, so fix generic atomics

On Wednesday 18 May 2016 16:10:45 David Howells wrote:
> cmpxchg_local() is not signed-value safe because on a 64-bit machine signed
> int arguments to it may be sign-extended to signed long _before_ begin cast
> to unsigned long.  This potentially causes comparisons to fail when dealing
> with negative values.
> 
> Fix the generic atomic functions that are implemented in terms of cmpxchg()
> to cast their arguments to unsigned int before calling cmpxchg().
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> 

Isn't the problem you describe something that cmpxchg() could prevent instead?

	Arnd