DF and signal handlers

[Aurelien Jarno]

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.

> 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.

> 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.

> before ever setting DF? Is setting DF Just A Bad Idea?

I don't think so. Not setting DF means less optimized code has to be used.

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