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Date:	Mon, 28 Jul 2014 20:09:02 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Alexei Starovoitov <ast@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Michel Dänzer <michel@...nzer.net>,
	Jakub Jelinek <jakub@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Debian GCC Maintainers <debian-gcc@...ts.debian.org>,
	Debian Kernel Team <debian-kernel@...ts.debian.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On 2014.07.28 at 10:27 -0700, Alexei Starovoitov wrote:
> On Mon, Jul 28, 2014 at 09:45:45AM -0700, Linus Torvalds wrote:
> > On Mon, Jul 28, 2014 at 5:26 AM, Frank Ch. Eigler <fche@...hat.com> wrote:
> > >
> > > Please note that the data produced by "-g -fvar-tracking" is consumed
> > > by tools like systemtap, perf, crash, and makes a significant
> > > difference to the observability of debug AND non-debug kernels.
> > 
> > Yeah, and compared to having a buggy kernel, I care exactly this much: "".
> 
> It's not pretty, but adding it unconditionally was the right thing to do.
> Black listing compiler versions is too fragile.
> Look at the flip side: now size of build dir will be much smaller :)

It shouldn't be too hard to implement a simple check for the bug in the
next release. Just compile the gcc/testsuite/gcc.target/i386/pr61801.c
testcase with -fcompare-debug. If gcc returns 0 then
-fvar-tracking-assignments could safely be enabled again.

Here's the testcase:

int a, b, c;
void fn1 ()
{
  int d;
  if (fn2 () && !0)
    {
      b = (
           {
           int e;
           fn3 ();
           switch (0)
           default:
           asm volatile("" : "=a"(e) : "0"(a), "i"(0));
           e;
           });
      d = b;
    }
  c = d;
}

-- 
Markus
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