RFR: 8373096: JFR leak profiler: path-to-gc-roots search should be non-recursive

[Robert Toyonaga]

On Thu, 4 Dec 2025 15:54:04 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> A customer reported a crash when producing a JFR recording with `path-to-gc-roots=true`. It was a native stack overflow that occurred during the recursive path-to-gc-root search performed in the context of PathToGcRootsOperation.
> 
> We try to avoid this by limiting the maximum search depth (DFSClosure::max_dfs_depth). That solution is brittle, however, since recursion depth is not a good proxy for thread stack usage: it depends on many factors, e.g., compiler inlining decisions and platform specifics. In this case, the VMThread's stack was too small.
> 
> This RFE changes the algorithm to be non-recursive. 
> 
> Note that as a result of this change, the order in which oop maps are walked per oop is reversed : last oops are processed first. That should not matter for the end result, however. The search is still depth-first.
> 
> Note that after this patch, we could easily remove the max_depth limitation altogether. I left it in however since this was not the scope of this RFE.
> 
> Testing:
> 
> - Tested manually with very small (256K) thread stack size for the VMThread - the patched version works where the old version crashes out
> - Compared JFR recordings from both an unpatched version (with a large enough VMThread stack size) and a patched version; made sure that the content of "Old Object Sample" was identical
> - Ran locally all jtreg tests in jdk/jfr
> - GHAs

src/hotspot/share/jfr/leakprofiler/chains/dfsClosure.cpp line 103:

> 101:     } else {
> 102:       if (_mark_bits->is_marked(pointee)) {
> 103:         return;

I think this improvement is a good idea!  But maybe this line should be replaced with a `continue`, otherwise we can terminate the DFS prematurely and skip evaluation of other chains extending from other references already pushed to the stack.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28659#discussion_r2600708367