[PATCH] mscoree: Implement VTable fixup for x86_64 architecture.

[Paul Gofman]

On 01/22/2016 09:47 PM, Vincent Povirk wrote:
>
>
> Mono has a lock around the JIT compiler, and it caches the functions
> it generates. So we can have multiple threads writing the same
> function pointers into the vtable (which should be fine as long as
> every other thread that reads it will see either the old value or the
> new one). Mono doesn't reference-count JITed functions, and I'm not
> sure if cleaning them up is even possible.
Oh, I see. So it sounds like there is no clear way to unload the
assembly and a DLL with it at all? Do you know if this the case with Win
.NET? I am curious because I was trying to locate the issue I get with
crash on exit in my test case. I am not filling it yet as bug report as
we discussed because it also has MFC (which functions are actually
crashing on exit), and I am not sure yet that it is not some issue
unrelated to Mono. But this happens only if .Net assembly was loaded
during the application run and I suspected that crashed could be
triggered by some incompatible unitialization.

>
> Non-locked write to fixup->done is also OK as long as no thread can
> incorrectly read the value as TRUE and return before the vtable is
> ready. If a thread incorrectly reads it as FALSE, it'll just do a bit
> of unnecessary work. Actually, I'm not sure that field is needed at
> all, it seems like overwriting the values in the vtable should be
> enough.
I think it is a sort of nearly theoretical question because it would be
really very hard to get this in practice. I think if to remove
fixup->done it would totally OK as you say. But if not AFAIK it is
possible that you get an older vtable value but newer fixup->done if
threads are at distinct CPUs (strong enough "fence" is required to
guarantee memory access order).


BTW I submitted bug report with my test case and patch to mono:
https://bugzilla.xamarin.com/show_bug.cgi?id=37913