RFR: 8332506: SIGFPE In ObjectSynchronizer::is_async_deflation_needed()

[David Holmes]

On Wed, 18 Dec 2024 15:33:43 GMT, Fredrik Bredberg <fbredberg at openjdk.org> wrote:

> This PR solves a division by zero problem, that according to the bug report happened in `ObjectSynchronizer::is_async_deflation_needed()`. As it turns out it really happened in the inlined  `monitors_used_above_threshold()` function. The problematic line looked like this:
> 
> `size_t monitor_usage = (monitors_used * 100LL) / ceiling;`
> 
> Unfortunately the `ceiling` value was increased every time there where too many deflations without any progress. This whould eventually lead to an overflow in the `ceiling` value, and in unlucky circumstances, it would become zero. Thus causing a division by zero crash.
> 
> This PR makes sure not to increase the `ceiling` value if `monitor_usage` is below the `MonitorUsedDeflationThreshold`.
> It also makes sure the ceiling value is never zero, and does not wrap around.
> 
> Tested okay in tier1-5 and `test/hotspot/jtreg/runtime/Monitor/MonitorUsedDeflationThresholdTest.java`

Changes requested by dholmes (Reviewer).

src/hotspot/share/runtime/synchronizer.cpp line 1281:

> 1279:       _no_progress_cnt >= NoAsyncDeflationProgressMax) {
> 1280:     double remainder = (100.0 - MonitorUsedDeflationThreshold) / 100.0;
> 1281:     size_t new_ceiling = ceiling + (size_t)((double)ceiling * remainder) + 1;

if you are looking fort the minimal fix for the division-by-zero problem, then I think simply fixing this line to avoid overflow will suffice:

size_t delta = (size_t)(ceiling * remainder) + 1;
if (ceiling > SIZE_MAX - delta) { // overflow
  ceiling = SIZE_MAX;  // Or some other positive limit
} else {
  ceiling += delta;
}

-------------

PR Review: https://git.openjdk.org/jdk/pull/22815#pullrequestreview-2513135911
PR Review Comment: https://git.openjdk.org/jdk/pull/22815#discussion_r1891011911