RFR(S): 8050972: Concurrency problem in PcDesc cache

Vladimir Kozlov vladimir.kozlov at oracle.com
Thu Jul 17 15:52:22 UTC 2014

First, comments needs to be fixed:

"The array elements must be volatile" but in changeset comments: "// Array MUST be volatile!"

Second, type name should be camel style (PcDescPtr).

Someone have to double check this volatile declaration. Your example is more clear for me than typedef.


On 7/17/14 8:19 AM, Doerr, Martin wrote:
> Hi Vladimir,
> the following line should also work:
> PcDesc* volatile _pc_descs[cache_size];
> But we thought that the typedef would improve readability.
> The array elements must be volatile, not the PcDescs which are pointed to.
> Best regards,
> Martin
> -----Original Message-----
> From: hotspot-dev [mailto:hotspot-dev-bounces at openjdk.java.net] On Behalf Of Vladimir Kozlov
> Sent: Donnerstag, 17. Juli 2014 17:09
> To: Lindenmaier, Goetz; hotspot-dev at openjdk.java.net
> Subject: Re: RFR(S): 8050972: Concurrency problem in PcDesc cache
> Hi Goetz,
> What is the reason for new typedef?
> Thanks,
> Vladimir
> On 7/17/14 1:54 AM, Lindenmaier, Goetz wrote:
>> Hi,
>> This webrev fixes an important concurrency issue in nmethod.
>> Please review and test this change.  I please need a sponsor.
>> http://cr.openjdk.java.net/~goetz/webrevs/8050972-pcDescConc/webrev-01/
>> This should be fixed into 8u20, too.
>> The entries of the PcDesc cache in nmethods are not declared as volatile, but they are accessed and modified by several threads concurrently. Some compilers (namely xlC 12 on AIX) duplicate some memory accesses to non-volatile fields. In this case, this has led to the situation that a thread had successfully matched a pc in the cache, but returned the reloaded value which was already overwritten by another thread.
>> Best regards,
>>     Martin and Goetz.

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