Re: [Gc] Pushing Registers for Collector Thread

Hi Jonathan,

First, note that GC_push_stack_for() exists only in win32. Other OSes are
organised a little differently.

But in general, the registers for the current thread are not supposed to be
pushed by GC_push_all_stacks because that has already been done elsewhere.

Look at GC_push_roots in mark_rts.c. The last thing is
calling GC_push_other_roots() if it is set. On posix and win32 there is a
default function that does the pushing of stacks and thread registers.

But immediately before that, GC_push_roots()
calls GC_push_regs_and_stack(), which is what is supposed to push the
registers for the current thread. It uses GC_with_callee_saves_pushed() in
machdep.c, which on win32 does a setjmp, which saves the registers in a
local structure in that function.

Perhaps this is not working for you, or you have the wrong #DEFINEs to make
it work?



On Sat, Sep 6, 2014 at 1:14 AM, Jonathan Chambers <[email protected]> wrote:

> Hello,
>
> Posting to the mailing list an issue one of our customers raised with us
> and on github. We (Unity) use bdwgc within the Mono VM. We are running an
> older version, but the issue highlighted still seems to persist in master.
>
> https://github.com/ivmai/bdwgc/issues/50
>
> I believe this may apply to more architectures/platforms, but at the
> moment I'll reference Win64. In GC_push_stack_for, all threads push their
> stack sections. However, when pushing register values the current thread is
> skipped:
>
> https://github.com/ivmai/bdwgc/blob/master/win32_threads.c#L1365
>
> This leads to a case where memory may be collected while still in use.
> Suppose an function allocates memory from the GC and it gets stored in a
> volatile register (RCX for example on Win64). No pointer on the stack or
> managed heap reference this allocation. Now a collection is triggered.
>
> If any subsequent functions in the call chain between the allocating
> function and GC_push_stack_for does use RCX, the original value should be
> pushed onto the stack and restored later as it's a volatile register. This
> stack reference will be found and marked since we push the current thread
> stack.
>
> However, if any subsequent functions in the call chain between the
> allocating function and GC_push_stack_for does *not* use RCX the reference
> will be not be marked and will be collected.
>
> On Windows/OSX this seems to always be the case. In pthread_stop_world.c I
> see calls to GC_save_regs_in_stack for SPARC and Itanium. Is there a reason
> this is not done/needed for all platforms?
>
> If this is indeed a bug, we can provide patches for all the platforms we
> support.
>
> Thanks,
> Jonathan
>
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