Integrated: 8268139: CDS ArchiveBuilder may reference unloaded classes

[Ioi Lam]

On Wed, 2 Jun 2021 21:31:55 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> During CDS dump, we enter a safepoint (e.g., `VM_PopulateDynamicDumpSharedSpace`) and run `ArchiveBuilder::gather_source_objs()` to collect all classes to be dumped. However, some of these classes may belong to custom class loaders that are no longer alive. While we are still inside the safepoint, concurrent GC operations may free these class loaders (see https://github.com/openjdk/jdk/pull/4286). Thus `ArchiveBuilder` may end up referencing unloaded classes.
> 
> The fix:
> 
> (1) Add a class `k` to ArchiveBuilder only if `k->class_loader_data()->is_alive()`
> 
> (2) All the classes added to `ArchiveBuilder` are taken from `DumpTimeSharedClassTable`. When a class is loaded, it's added to this table. When a class is unloaded, it's removed from this table (via `SystemDictionaryShared::remove_dumptime_info`). This table is protected by `DumpTimeTable_lock`. To make sure that this table is not changed during the CDS safepoint, we grab this lock before calling `ArchiveBuilder::gather_source_objs()`, and release this lock only when the CDS dump is finished. As a result, in step (1) above, when we call `k->class_loader_data()->is_alive()`, we are sure that `k` itself has not been deallocated and is still a valid pointer.
> 
> Testing -- Mach5 tiers 1-4 in progress

This pull request has now been integrated.

Changeset: 91f9adc5
Author:    Ioi Lam <iklam at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/91f9adc5f55b42d30e46d99e22a8d0567fa5c146
Stats:     23 lines in 4 files changed: 15 ins; 0 del; 8 mod

8268139: CDS ArchiveBuilder may reference unloaded classes

Reviewed-by: coleenp, ccheung

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PR: https://git.openjdk.java.net/jdk/pull/4322