linux-kernel - Re: question about RCU dynticks

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:	Tue, 5 May 2015 15:00:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, williams@...hat.com,
	Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: question about RCU dynticks_nesting

On Tue, May 05, 2015 at 05:34:46AM -0700, Paul E. McKenney wrote:
> On Tue, May 05, 2015 at 12:53:46PM +0200, Peter Zijlstra wrote:
> > On Mon, May 04, 2015 at 12:39:23PM -0700, Paul E. McKenney wrote:
> > > But in non-preemptible RCU, we have PREEMPT=n, so there is no preempt
> > > counter in production kernels.  Even if there was, we have to sample this
> > > on other CPUs, so the overhead of preempt_disable() and preempt_enable()
> > > would be where kernel entry/exit is, so I expect that this would be a
> > > net loss in overall performance.
> > 
> > We unconditionally have the preempt_count, its just not used much for
> > PREEMPT_COUNT=n kernels.
> 
> We have the field, you mean?  I might be missing something, but it still
> appears to me thta preempt_disable() does nothing for PREEMPT=n kernels.
> So what am I missing?

There's another layer of accessors that can in fact manipulate the
preempt_count even for !PREEMPT_COUNT kernels. They are currently used
by things like pagefault_disable().
--
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/