Re: The 2.0.9 VM cores in enqueue (threads.c:309) -- partial fix, patch attached

[Andrew Gaylard]

On 04/29/13 12:10, Mark H Weaver wrote:
> Hi Andrew,
> On 28 April 2013 03:57, Andrew Gaylard <address@hidden> wrote:
> > > > Those 0x304 values look dodgy to me, and explain why the
> > > > SCM_SETCDR causes an invalid memory access.
> > (gdb) p *SCM2PTR(q)
> > $26 = {word_0 = 0x304, word_1 = 0x1039c4c20}
> What's happening here is that the wait queue (m->waiting in fat_mutex)
> is somehow getting corrupted.  The code above ('enqueue' in threads.c)
> is trying to add a new element to the queue.  The queue is represented
> as a pair whose CDR is the list of items in the queue, and whose CAR
> points to the last pair of that list.  Somehow, the CAR is becoming null
> even though the CDR is non-empty.  This should never happen.
>
> I looked through the relevant code, and it's not obvious to me how this
> could happen.  The only functions I see that manipulate this queue are
> 'enqueue', 'remqueue', and 'dequeue', all static functions in threads.c.
> As far as I can see, these functions maintain the invariant that the CAR
> is null if and only if the CDR is null.  All queue manipulation is done
> between SCM_CRITICAL_SECTION_START and SCM_CRITICAL_SECTION_END (defined
> in async.h) which lock a single global pthread mutex.
>
> Any ideas?
>
>       Thanks,
>         Mark
Hi,

I've had some more time to look into this problem, and now have a 
partial fix.

The problem does not occur on Linux x86 or x86_64 (Ubuntu-12.04).
The problem always occurs on Solaris-10u9, both x86_64 and SPARC.

The problem is always the segmentation fault trying to write to write
to 0x30c, at threads.c:309.  Inspection of the remqueue function shows
that the logic is not correct when removing the last entry in the queue.

The patch attached helps -- my code runs for much longer, but doesn't crash.
However it now hangs somewhere else (which may be an unrelated problem).

I'd be grateful for any feedback.
--
Andrew

fix-guile-thread-remqueue.patch
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