On Sunday, December 30, 2012 2:13:23 AM UTC+1, Nils Bruin wrote: > > > > Maybe the problem is with endomorphism rings, because we have the > domain > > > and codomain pointing to the same parent, that's a nice culprit for a > > > superfluous decref. > > I spent some quality time with gdb and this failing doctest. The good > news: memory layout is deterministic, so you can just set a "watch" on > a memory location and get a break every time that location gets > touched. I've analyzed what goes wrong (at least for me) to a certain > degree: > > PyMalloc uses pools for fixed sized memory blocks. Unused blocks in a > page are linked together in a "freelist". The segfault arises from > interpreting a non-pointer value as a pointer value. The "1" that ends > up there happens to be a refcount of a string. I got the strong > impression that that same block also got allocated as a "tuple", which > has a "GC block" in front of it. The 1 should wreak havoc there as > well, but apparently never does. It may well be that this block got > inserted twice into a freelist and hence gets allocated twice, which > later on messes up the memory management. > > Just prior to the activity that I thought looked like a double free, I > noticed that this block is indeed used for allocation of > sage.categories.modules.Modules.HomCategory.parent_class. This is > happening around the doctest > > sage: from sage.modules.module import Module > sage: M = Module(ZZ); M > sage: M.endomorphism_ring() > > and some free activity afterwards did have weakref.callback stuff in > its stack trace. > > I had good hopes that with a --without-pymalloc build, we'd catch a > double free like this red handed, but no such luck (I cannot reproduce > the problem at all on debug builds).
Me neither, too bad. At least it got us to have a working Python debug build. > My afternoon with gdb has > horribly little to show for, but at least my observations seem in line > with what JP has found. > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To post to this group, send email to sage-devel@googlegroups.com. To unsubscribe from this group, send email to sage-devel+unsubscr...@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel?hl=en.
