linux-kernel - Re: [PATCH 3.15 33/37] Fix gcc-4.9.0 miscompilation of load

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Date:	Tue, 5 Aug 2014 08:19:15 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Josh Boyer <jwboyer@...oraproject.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jakub Jelinek <jakub@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>,
	Michel Dänzer <michel@...nzer.net>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>
Subject: Re: [PATCH 3.15 33/37] Fix gcc-4.9.0 miscompilation of
 load_balance() in scheduler

On Tue, Aug 05, 2014 at 07:31:22AM -0400, Josh Boyer wrote:
> On Wed, Jul 30, 2014 at 11:47 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > On Tue, Jul 29, 2014 at 11:53 PM, Jakub Jelinek <jakub@...hat.com> wrote:
> >>
> >> IMNSHO this is a too big hammer approach.  The bug happened on a single
> >> file only (right?)
> >
> > Very dubious. We happened to see it in a single case, and _maybe_ that
> > was the only one in the whole kernel. But it's much more likely that
> > it wasn't - it's not like the code in question was even all that
> > unusual (just a percpu access triggering an asm - but we have tons of
> > asms in the kernel).
> >
> > I'd argue that we were very lucky to get the problem happening
> > reliably enough for a couple of people who then cared enoiugh to do
> > good bug reports (considering that it needed an interrupt in *just*
> > the right place) that we could debug it at all. In some code that gets
> > run much less than the scheduler, it could easily have been one of
> > those "people report it once in a blue moon, looks like memory
> > corruption".
> >
> > Now, it would be interesting to hear if there is something very
> > special that made that instruction scheduling bug trigger just for
> > 4.9.x, or if there is something else that made it very particular to
> > that code sequence. But in the absence of good reasoning to the
> > contrary, I'd much rather say "let's just avoid the bug entirely".
> >
> > And that's partly because we really don't care that much about the
> > debug info. Yes, it gets used, but it's not *that* common, and the
> > last time the issue of debug info sucking up tons of resources came
> > up, the biggest users were people who just wanted line information for
> > oopses. Yes, there are people running kgdb etc, but on the whole it's
> > rare, and quite frankly, from everything I have _ever_ seen, that's
> > not how the real kernel bugs are ever really discovered. So the kind
> > of debug information that the variable tracking logic adds just isn't
> > all that important for the kernel.
> 
> Sorry to bring this back up after the fact, but it's important for a
> number of things in various distros.  I don't disagree it should be
> disabled by default, but making it unconditional is going to force the
> distributions that care about perf, systemtap, and debuggers to
> manually revert this.  That deviation is concerning because the
> upstream kernel won't easily be buildable the same way distros build
> it.

Why does this patch affect perf and other debuggers?

thanks,

greg k-h
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