linux-kernel - Re: bit fields && data tearing

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:	Fri, 5 Sep 2014 21:52:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Michael Cree <mcree@...on.net.nz>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Laight <David.Laight@...LAB.COM>,
	Jakub Jelinek <jakub@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Miroslav Franc <mfranc@...hat.com>,
	Richard Henderson <rth@...ddle.net>,
	linux-alpha@...r.kernel.org
Subject: Re: bit fields && data tearing

On Fri, Sep 05, 2014 at 11:31:09AM -0700, Paul E. McKenney wrote:
> compiler: Allow 1- and 2-byte smp_load_acquire() and smp_store_release()
> 
> CPUs without single-byte and double-byte loads and stores place some
> "interesting" requirements on concurrent code.  For example (adapted
> from Peter Hurley's test code), suppose we have the following structure:
>     
>     	struct foo {
>     		spinlock_t lock1;
>     		spinlock_t lock2;
>     		char a; /* Protected by lock1. */
>     		char b; /* Protected by lock2. */
>     	};
>     	struct foo *foop;
>     
> Of course, it is common (and good) practice to place data protected
> by different locks in separate cache lines.  However, if the locks are
> rarely acquired (for example, only in rare error cases), and there are
> a great many instances of the data structure, then memory footprint can
> trump false-sharing concerns, so that it can be better to place them in
> the same cache cache line as above.
> 
> But if the CPU does not support single-byte loads and stores, a store
> to foop->a will do a non-atomic read-modify-write operation on foop->b,
> which will come as a nasty surprise to someone holding foop->lock2.  So we
> now require CPUs to support single-byte and double-byte loads and stores.
> Therefore, this commit adjusts the definition of __native_word() to allow
> these sizes to be used by smp_load_acquire() and smp_store_release().

So does this patch depends on a patch that removes pre EV56 alpha
support? I'm all for removing that, but I need to see the patch merged
before we can do this.
--
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/