linux-kernel - Re: [PATCH] "volatile considered harmful" document

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]

Message-ID: <46437416.4070401@zytor.com>
Date:	Thu, 10 May 2007 12:35:50 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jonathan Corbet <corbet@....net>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Randy Dunlap <randy.dunlap@...cle.com>
Subject: Re: [PATCH] "volatile considered harmful" document

Jonathan Corbet wrote:
> OK, here's an updated version of the volatile document - as a plain text
> file this time.  It drops a new file in Documentation/, but might it be
> better as an addition to CodingStyle?
> 
> Comments welcome,

I have found one use of volatile which I consider legitimate: pointers
to data structures read or written by I/O devices in coherent memory
(typically pci_coherent memory.)  This is local to device drivers, but
as far as I can tell, the use of volatile here is legitimate, although
arguably it will be redundant in > 90% of all cases due to the
incidental presence of other memory barriers.

In Ethernet drivers, for example, it is common for the network card to
maintain a pointer in host memory the the latest descriptor written; you
will generally have a loop of the form:

	while ((this_pointer = *pointer_ptr) > my_last_pointer) {
		for (pkt = my_last_pointer; pkt < this_pointer; pkt++)
			receeive_packet(pkt);
		my_last_pointer = this_pointer;
	}

pointer_p can then be a volatile pointer into said coherent memory.

	-hpa

		
-
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/