* Re: [BELATED CORE TOPIC] context tracking / nohz / RCU state
[not found] ` <20150812143819.GE21542@lerouge>
@ 2015-08-12 15:59 ` Paul E. McKenney
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From: Paul E. McKenney @ 2015-08-12 15:59 UTC (permalink / raw)
To: Frederic Weisbecker
Cc: Andy Lutomirski, ksummit-discuss@lists.linuxfoundation.org,
linux-kernel@vger.kernel.org, Christoph Lameter, Ingo Molnar,
Thomas Gleixner, H. Peter Anvin, Peter Zijlstra, Chris Metcalf,
Rik van Riel, benh, will.deacon, linux-arch
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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