[crac] RFR: RFC: -XX:CPUFeatures=0xnumber for CPU migration [v3]

[Jan Kratochvil]

On Thu, 2 Feb 2023 06:09:18 GMT, Jan Kratochvil <duke at openjdk.org> wrote:

>> src/hotspot/os/linux/os_linux.cpp line 6177:
>> 
>>> 6175:   assert(sig == RESTORE_SIGNAL, "got what requested");
>>> 6176: 
>>> 6177:   linux_ifunc_reset();
>> 
>> Assuming we're freezing all the threads to allow resetting the linux_ifuncs, it would be good to comment explicitly about why that's the right approach and/or necessary.
>> 
>> Also, what if we stopped a thread that was about to be call one of these ifuncs - that had already loaded the old address into a register - before we did the reset?  Wouldn't it still have the wrong value and with the right (wrong?) thread scheduling, still abort after the restore?
>> 
>> Basically, I think there is probably a race here still.
>
> You are right, there is a race, I will fix that, thanks.

The race should be fixed now. Although still all threads of OpenJDK should be at a safepoint but in practice they are not, why so?

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PR: https://git.openjdk.org/crac/pull/41