At Sat, 9 May 2020 07:18:01 +0200, Dominik Pantůček wrote:
> would this be enough to open an issue for that?
> (gdb) info threads
>    Id   Target Id                                        Frame
> * 1    Thread 0x7ffff7c1b300 (LWP 19075) "tut22.rkt" 
> mark_backpointers (gc=gc@entry=0x5555559d10c0) at 
> ../../../racket/gc2/newgc.c:4078

Yes, this might identify the problem. Being stuck in a linked-list
iteration often means that there was a race updating the list.

The GC's write barrier is implemented by write-protecting pages and
handling SIGSEGV to record the modification (and remove write
protection until the next GC). If that handler is called in different
future threads, though, then there's currently a race on the list of
modified pages.

This race doesn't happen with places, because different places have
different GC instances. And it won't happen on Mac OS, because the
fault is handled at the Mach layer and routes exceptions for all
threads to a single handler thread.

I'll add a lock at lines 1092-1096 of "newgc.c", and we'll see if that

Thanks very much for your help!

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