RFR: 8275049: [ZGC] missing null check in ZNMethod::log_register

[Erik Österlund]

On Mon, 11 Oct 2021 12:23:03 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

> The VM crashes while trying to read (*p)->klass() in "ZNMethod::log_register" on PPC64. We need a null check. See JBS for details.

The oops we read here are written in c1_Runtime, while holding the Patching_lock. However, the Patching_lock is not held when registering. In other words, someone could be writing an oop while we are reading it in this loop, due to C1 patching. If the loads re-order in here, we may crash the VM. Since we have plain loads, the compiler is free to re-order. I think the solution I would go with, is to use the CompiledICLocker instead where we patch the code in C1. That ends up taking the per-nmethod lock, that we can hold while logging this. That makes sure that accessing the oops implies mutual exclusion. Then we can remove the Patching_lock, since it is only ever used in that one place.

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Changes requested by eosterlund (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5892