RFR(S): 8134493: Cleaning inline caches of unloaded nmethods should be done in sweeper
tobias.hartmann at oracle.com
Thu Aug 27 13:05:44 UTC 2015
thanks for looking at this.
On 27.08.2015 12:26, Doerr, Martin wrote:
> one question about always avoiding transition stubs for unloaded nmethods in cleanup_inline_caches() came into my mind when taking a second look at the change.
> When we transition to zombie state, this should be safe because it's guaranteed that no activations exist.
> However, cleanup_inline_caches() is used at other places, too.
> The comment about the unloaded state only says "// there should be no activations". Is it guaranteed that there is no activation of an nmethod which is in unloaded state?
> If not, the change may have undesired side effects.
I think it is guaranteed that unloaded nmethods are not on the stack and it should be save to clean their ICs without a transition stub. We also rely on this assumption in the sweeper by directly flushing unloaded OSR nmethods (see line 644 in sweeper.cpp ).
Maybe someone from the GC team (CC'ed) can clarify this.
> Besides this, the change looks good to me.
> Best regards,
> -----Original Message-----
> From: Tobias Hartmann [mailto:tobias.hartmann at oracle.com]
> Sent: Donnerstag, 27. August 2015 09:10
> To: hotspot-compiler-dev at openjdk.java.net
> Cc: Doerr, Martin
> Subject:  RFR(S): 8134493: Cleaning inline caches of unloaded nmethods should be done in sweeper
> please review the following patch.
> This is a follow up on JDK-8075805  which modified CodeCache::gc_epilogue() to clean the ICs of unloaded nmethods as well. The problem is that this code is executed at a safepoint and may affect safepoint duration. The other changes of JDK-8134493 are fine.
> We do the cleaning of unloaded nmethods at the unloaded -> zombie transition in the sweeper. I also modified nmethod::cleanup_inline_caches() to not emit any transition stubs if the nmethod is already dead.
> As Martin Doerr pointed out in another thread, we have to be careful with accessing CompiledIC::cached_metadata() of unloaded nmethods. For example, the following scenario may happen (IC of A references B):
> state of A state of B
> S [not-on-stack]
> S zombie
> Now the IC of A still references the unloaded nmethod B and is_call_to_compiled() will access the unloaded metadata. I fixed this by checking caller->is_alive().
>  https://bugs.openjdk.java.net/browse/JDK-8075805
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