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Posted to pr@cassandra.apache.org by "yifan-c (via GitHub)" <gi...@apache.org> on 2026/10/08 23:43:49 UTC
Re: [PR] CASSANDRA-21714: Fix use-after-free when Zstd dictionary compressor cache evicts concu… [cassandra]
yifan-c commented on code in PR #5206:
URL: https://github.com/apache/cassandra/pull/5206#discussion_r4225248148
##########
src/java/org/apache/cassandra/io/compress/ZstdDictionaryCompressor.java:
##########
@@ -146,6 +146,12 @@ public int uncompress(byte[] input, int inputOffset, int inputLength, byte[] out
return super.uncompress(input, inputOffset, inputLength, output, outputOffset);
}
+ // Hold our own reference for the duration of the call: the cache's dictionaryRef can be released by
+ // eviction concurrently with this call, and tidy() must not run while a context is still in use.
+ Ref<ZstdCompressionDictionary> ref = dictionary.tryRef();
Review Comment:
I had the JFR recording from a 30 minutes run with the benchmark.
The loop should generate roughly 3 billion `Ref` objects. According to the JFR recording, only 1144 `PhantomReference` are found during GC, meaning almost none `PhantomReference` were still alive when a GC looked.
```
❯ $JAVA_HOME/bin/jfr print --json --events jdk.GCReferenceStatistics reaper30.jfr \
| jq '[.recording.events[].values | select(.type=="Phantom reference") | .count] | {gcs: length, total: add, max: max}'
{
"gcs": 4825,
"total": 1114,
"max": 991
}
```
The CPU usage from the reaper thread is negligible, close to 0. See the output below — the cpu time grows steadily, and only used `595.05 ms` out of `1807 second`, roughly 0.03% CPU.
```
❯ $JAVA_HOME/bin/jfr print --events jdk.ThreadDump reaper30.jfr | grep -E '^"Reference-Reaper"'
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=24.47ms elapsed=58.98s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=46.16ms elapsed=119.24s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=66.75ms elapsed=179.52s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=88.90ms elapsed=239.80s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=110.15ms elapsed=300.06s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=130.58ms elapsed=360.38s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=153.11ms elapsed=420.67s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=174.27ms elapsed=480.94s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=192.87ms elapsed=541.20s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=212.78ms elapsed=601.42s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=232.02ms elapsed=661.69s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=252.72ms elapsed=721.97s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=272.00ms elapsed=782.24s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=290.43ms elapsed=842.52s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=307.97ms elapsed=902.78s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=325.60ms elapsed=963.12s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=346.07ms elapsed=1023.41s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=366.70ms elapsed=1083.67s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=384.33ms elapsed=1143.93s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=404.19ms elapsed=1204.19s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=423.77ms elapsed=1264.46s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=442.15ms elapsed=1324.74s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=462.17ms elapsed=1385.01s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=481.56ms elapsed=1445.30s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=498.18ms elapsed=1505.56s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=515.78ms elapsed=1565.86s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=535.11ms elapsed=1626.20s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=553.58ms elapsed=1686.46s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=574.77ms elapsed=1746.73s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
"Reference-Reaper" #52 [43291] daemon prio=5 os_prio=31 cpu=595.04ms elapsed=1807.02s tid=0x0000007c926f4e10 nid=43291 waiting on condition [0x000000016fe1a000]
```
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