[LUCENE-6197] ConcurrentMergeScheduler should not stall its own merge threads!

• Type: Bug
• Status: Closed
• Priority: Blocker
• Resolution: Fixed
• Fix Version/s: 5.0, 5.1, 6.0
• Lucene Fields: New
• Assignee: Michael McCandless
• Reporter: Michael McCandless
• Created: 23/Jan/15 16:57
• Updated: 28/Aug/22 14:23
• Resolved: 24/Jan/15 15:18

Description

http://build-eu-00.elasticsearch.org/job/lucene_trunk_linux_java8_64_analyzers/25834
uncovered this issue.

I accidentally introduced this with auto-IO-throttle (LUCENE-6119)
... the CMS.maybeStall method, which is supposed to block "segment
producing" threads so indexing slows down when merges cannot keep up,
can now sometimes block its own merge threads. This happens when the merge
thread re-invokes CMS.merge after it finishes, so new merges can kick
off.

This is really silly, since merge threads are not segment producers,
but rather the "messengers", spawned by the true segment producers,
so CMS should not shoot the messenger here.

I think it's also possible this could lead to deadlock in CMS, but I'm
not certain, and I couldn't provoke it.

I'd like to fix this for 5.0 because it was first introduced in 5.0
and not yet released...