linux-kernel - Re: [BELATED CORE TOPIC] context tracking / nohz / RCU state

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Date:	Wed, 12 Aug 2015 08:59:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"ksummit-discuss@...ts.linuxfoundation.org" 
	<ksummit-discuss@...ts.linuxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Rik van Riel <riel@...hat.com>, benh@...nel.crashing.org,
	will.deacon@....com, linux-arch@...r.kernel.org
Subject: Re: [BELATED CORE TOPIC] context tracking / nohz / RCU state

On Wed, Aug 12, 2015 at 04:38:21PM +0200, Frederic Weisbecker wrote:
> On Tue, Aug 11, 2015 at 11:33:12AM -0700, Paul E. McKenney wrote:
> > On Tue, Aug 11, 2015 at 10:49:36AM -0700, Andy Lutomirski wrote:
> > > Some arches may need:
> > > 
> > > i_am_lame_and_forgot_my_previous_context()
> > > 
> > > x86 will soon (4.3 or 4.4, depending on how my syscall cleanup goes)
> > > no longer need that.
> > > 
> > > Paul says that some arches need something that goes straight from IRQ
> > > to user mode (?) -- sigh.
> > 
> > Straight from IRQ to process-level kernel mode.  I ran into this in
> > late 2011, and clearly should have documented exactly what code was
> > doing this.  Something about invoking system calls from within the
> > kernel on some architectures.
> > 
> > Hey, if no architectures do this anymore, I could simplify RCU a bit!  ;-)
> 
> That issue has always been a bit foggy to me :-)
> 
> We never really stated what exactly the issue was. Just performing syscalls
> from kernel mode shouldn't fiddle with the dynticks count.
> 
> IIUC, the issue was that some IRQs triggered and never returned. But we
> certainly can't remove the safety code without clearly identifying the
> issue...

This was not a theoretical problem -- there were real failures.

But yes, the safety code is there and seems to work OK, so I do need
confirmation of a change before removing it.  I do recall someone
arguing that the half-interrupts should go away, but I never did hear
that they really did go away.

Adding linux-arch in the hope that someone can say for sure.

							Thanx, Paul

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