Bug#469058: DF and signal handlers

[Aurelien Jarno]

reassign 469058 linux-2.6,gcc-4.3
thanks

On Wed, Mar 05, 2008 at 04:10:53PM +0100, Aurelien Jarno wrote:
> On Wed, Mar 05, 2008 at 03:48:05PM +0200, Nikodemus Siivola wrote:
> > On 3/5/08, Aurelien Jarno <aurelien at aurel32.net> wrote:
> > > Nikodemus Siivola a écrit :
> > >
> > > > On 3/5/08, Debian Bug Tracking System <owner at bugs.debian.org> wrote:
> > >  >
> > >  >> tag 469058 + patch
> > >  >>  Bug#469058: sbcl doesn't reset direction flag upon exit
> > >  >>  There were no tags set.
> > >  >>  Tags added: patch
> > >  >
> > >  > Thanks for the patch, but... while I agree that it is good to change
> > >  > SBCL to reset the direction flag every time it is diddled, instead of
> > >  > just before calling C, I don't think SBCL is actually at fault here.
> > >  >
> > >  >  1. SBCL does actually reset DF before any call to foreign (GCC generated) code.
> > >  >     See line 236 in src/compiler/x86/c-call.lisp, and line 125 in
> > >  >     src/runtime/x86-assem.S.
> > >  >
> > >  >     (It is possible I'm missing out a call-path here, but even so, read on and
> > >  >     see if my fears are unfounded or not.)
> > >  >
> > >  >  2. If the problem was due to a foreign call, it should be deterministic.
> > >  >
> > >  >  3. If the problem was due to _returning_ to main(), it should be deterministic.
> > >
> > >
> > > Looks correct.
> > >
> > >
> > >  > What I suspect is actually going on (especially considering your
> > >  > statement that compiling signals/ with 4.2 avoided the issue) is that
> > >  > a signal handler is entered while DF is set.
> > >
> > >
> > > What I am sure is that sigemptyset() from the glibc is called with the
> > > direction flag set, and that should not happen.
> > 
> > Right.
> > 
> > I'm about to merge a patch to SBCL based on yours, which moves all DF
> > resets to immediate vicinity of STDs for easier auditing, and removed
> > the then-unnecessary CLD instructions from foreign call sequences.
> > This will fix them symptoms, and be good for SBCL, but I think the
> > underlying problem is still there in signal handling. :/
> > 
> > >  > If this is the case, then clearing it right after each REP loop where
> > >  > SBCL uses it just makes seeing the bug much more unlikely -- but not
> > >  > impossible in the presence of async signals.
> > >
> > >
> > > Seems correct, though I have made half a dozen of build here, without
> > > any problem.
> > 
> > That is not too suprising: the are normally no asynch signals
> > delivered during the build, but SIGSEGV is a regular occurance (it is
> > used by the GC), so SIGSEGV handlers may have been seeing the DF set.
> > 
> > What _is_ strange is that this appears to have been random. (At least
> > all the reporters seemed to characterize it as semirandom behaviour.)
> > Multiple builds from the same source with the same host compiler
> > should have essentially identical GC characteristics.
> > 
> > >  > If so, this may also explain some _very_ hard to reproduce faults we
> > >  > have seen over the years: using a pre 4.3-GCC compiled libc, a signal
> > >  > at an in opportune moment in the middle of a REP loop could clear DF!
> > >  > Yikes!
> > >  >
> > >  > I'm not sure what is The Right Thing here, though. Should SBCL (and
> > >  > _any_ program that ever sets DF!) save, clear, and restore DF in its
> > >  > signal handlers? Should libc/kernel do that? Should signals be blocked
> > >
> > >
> > > I currently have no idea about that.
> > 
> > I'll see if I can cook up a small test-case using async signals. (One
> > that doesn't need SBCL so that it can be passed to upstream libc /
> > kernel people if necessary without too much friction.)
> > 

That's definitively a kernel/gcc-4.3 problem, I have reported it
upstream: http://lkml.org/lkml/2008/3/5/207

I am therefore reassigning the bug to those packages.

-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32 at debian.org         | aurelien at aurel32.net
   `-    people.debian.org/~aurel32 | www.aurel32.net