Re: Bug#561203: threads and fork on machine with VIPT-WB cache

[dann frazier <dannf@debian.org>]

On Wed, Jun 02, 2010 at 01:16:01PM -0400, John David Anglin wrote:
> On Wed, 02 Jun 2010, Modestas Vainius wrote:
> 
> > Hello,
> > 
> > this bug [1] is back to the "very common" department with eglibc 2.11 (libc6-
> > dev_2.11.1-1) builds. Majority of KDE applications are failing to build on 
> > hppa again. Is there really nothing what could be done to fix it?
> 
> I will just say it is very tricky.  I think a fix is possible (arm and mips
> had similar cache problems) but the victim replacement present in PA8800/PA8900
> caches makes the problem especially difficult  for hardware using these
> processors.
> 
> I have spent the last few months testing various alternatives and have
> now done hundreds of kernel builds.  I did post some experimental patches
> that fix the problem on UP kernels.  However, the problem is not resolved
> for SMP kernels.

Note that Debian's buildds run a UP kernel, so as soon as those fixes
go upstream we can pull them in. Thanks for all your work here!

> The minifail test is a good one to demonstrate the problem.  Indeed,
> a very similar test was given in the thread below:
> http://readlist.com/lists/vger.kernel.org/linux-kernel/54/270861.html
> 
> This thread also discusses the PA8800 problem:
> http://readlist.com/lists/vger.kernel.org/linux-kernel/54/271417.html
> 
> I currently surmise that we have a problem with the cache victim
> replacement, although the cause isn't clear.  I did find recently
> that the cache prefetch in copy_user_page_asm extends to the line
> beyond the end of the page, but fixing this doesn't resolve the problem.
> 
> I am still experimenting with using equivalent aliasing.  It does
> help to flush in ptep_set_wrprotect.
> 
> Dave

-- 
dann frazier