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Advance the trace once in ArenaStartCollect before releasing the arena #322

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Advance the trace once in ArenaStartCollect before releasing the arena #322

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11 changes: 11 additions & 0 deletions code/traceanc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -634,6 +634,17 @@ Res ArenaStartCollect(Globals globals, TraceStartWhy why)
res = TraceStartCollectAll(&trace, arena, why);
if(res != ResOK)
goto failStart;

/* The trace has been flipped, so the mutator may hit the read
* barrier on a grey segment as soon as the arena is released.
* TraceRankForAccess needs the trace to be past its initial band,
* RankAMBIG, but the band only advances in traceFindGrey.
* ArenaRelease advances the trace only if a poll is due, so advance
* it once here, as TracePoll and ArenaStep do after starting a
* trace. Afterwards either the band is past RankAMBIG or no
* segment is grey for the trace. */
TraceAdvance(trace);

ArenaRelease(globals);
return ResOK;

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