linux-kernel - Re: [PATCH v2] sched, timer: Use atomics for thread_group

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: <1425336200.5304.84.camel@j-VirtualBox>
Date:	Mon, 02 Mar 2015 14:43:20 -0800
From:	Jason Low <jason.low2@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Scott Norton <scott.norton@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	jason.low2@...com
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability

On Mon, 2015-03-02 at 13:44 -0800, Linus Torvalds wrote:
> On Mon, Mar 2, 2015 at 1:16 PM, Jason Low <jason.low2@...com> wrote:
> >
> > In original code, we set cputimer->running first so it is running while
> > we call update_gt_cputime(). Now in this patch, we swapped the 2 calls
> > such that we set running after calling update_gt_cputime(), so that
> > wouldn't be an issue anymore.
> 
> Hmm. If you actually care about ordering, and 'running' should be
> written to after the other things, then it might be best if you use
> 
>    smp_store_release(&cputimer->running, 1);
> 
> which makes it clear that the store happens *after* what went before it.
> 
> Or at least have a "smp_wmb()" between the atomic64 updates and the
> "WRITE_ONCE()".
> 
> I guess that since you use cmpxchg in update_gt_cputime, the accesses
> end up being ordered anyway, but it might be better to make that thing
> very explicit.

Yeah, I suppose the extra (smp_mb or smp_wmb) might add more overhead
but since this is not a common code path anyway, it would be worth
adding it to make things more clear.

--
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/