linux-kernel - Re: why are some atomic_t's not volatile, while most are?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 08 Aug 2007 02:47:53 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Chris Friesen <cfriesen@...tel.com>
CC:	Zan Lynx <zlynx@....org>, Jerry Jiang <wjiang@...ilience.com>,
	"Robert P. J. Day" <rpjday@...dspring.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: why are some atomic_t's not volatile, while most are?

Chris Friesen wrote:
> Chris Snook wrote:
> 
>> This is not a problem, since indirect references will cause the CPU to 
>> fetch the data from memory/cache anyway.
> 
> Isn't Zan's sample code (that shows the problem) already using indirect 
> references?

Yeah, I misinterpreted his conclusion.  I thought about this for a 
while, and realized that it's perfectly legal for the compiler to re-use 
a value obtained from atomic_read.  All that matters is that the read 
itself was atomic.  The use (or non-use) of the volatile keyword is 
really more relevant to the other atomic operations.  If you want to 
guarantee a re-read from memory, use barrier().  This, incidentally, 
uses volatile under the hood.

	-- Chris
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/