RFR (9) 8185133: Reference pending list root might not get marked

[Erik Österlund]

Hi Mikael,

Looks good.

/Erik

On 2017-07-28 14:50, Mikael Gerdin wrote:
> Hi all,
>
> Please review this fix to a tricky reference processing / conc marking 
> bug affecting G1 in 9.
>
> The bug occurs when a weak reference WR is promoted to old and 
> discovered during an initial mark pause. The WR is the referent of a 
> soft reference SR. The concurrent reference processor determines that 
> SR should be treated as a weak reference due to shortage of memory and 
> now WR is reachable only from the reference pending list but not 
> explicitly marked in the bitmap since objects promoted during the 
> initial mark pause are not marked immediately.
>
> The reason we are not saved by the SATB pre-barrier here is that 
> clearing of the referent field of a reference object does not trigger 
> the pre-barrier (and that would kind of defeat its purpose).
>
> Before JDK-8156500 this worked because the reference pending list was 
> a static field in the Reference class and the reference class was 
> scanned during concurrent marking, so we would never lose track of the 
> pending list head.
>
> My suggested fix is to explicitly mark the reference pending list head 
> oop during initial mark, after the reference enqueue phase.
> This mirrors how other roots are handled in initial mark, see 
> G1Mark::G1MarkPromotedFromRoots.
>
> Webrev: http://cr.openjdk.java.net/~mgerdin/8185133/webrev.0
> Bug: https://bugs.openjdk.java.net/browse/JDK-8185133
>
> Testing: JPRT, tier2-5 gc tests, a LOT of runs of the failing test.
>
> Many thanks to Kim and Erik Ö for discussions around this issue!
>
> Thanks
> /Mikael