8216987: ciMethodData::load_data() unpacks MDOs with non-atomic copy

[Erik Österlund]

Hi Martin,

Thanks for the review.

/Erik

On 2019-01-14 16:30, Doerr, Martin wrote:
> Hi Erik,
>
> this looks good.
>
> Best regards,
> Martin
>
>
> -----Original Message-----
> From: hotspot-compiler-dev <hotspot-compiler-dev-bounces at openjdk.java.net> On Behalf Of Erik Österlund
> Sent: Montag, 14. Januar 2019 16:18
> To: hotspot compiler <hotspot-compiler-dev at openjdk.java.net>
> Subject: 8216987: ciMethodData::load_data() unpacks MDOs with non-atomic copy
>
> Hi,
>
> The ciMethodData::load_data() member function copies a raw MDO to the
> compiler mirror of said MDO. However, the copy is performed using a
> non-atomic copy function, despite being updated concurrently. This could
> potentially cause word tearing when reading metadata pointers, causing
> the VM to crash... in theory.
>
> While this is not a problem when unpacking the extra data section,
> because it is done under a lock, the same can not be said about the rest
> of the MDO. So it should either be protected by a lock, or use an atomic
> copy function instead.
>
> This patch adds an extra seat belt by performing atomic heap word copy
> instead.
>
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8216987/webrev.00/
>
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8216987
>
> Thanks,
> /Erik