RFR: JDK-8282405: Make thread resource areas signal safe [v3]

[David Holmes]

On Wed, 2 Mar 2022 10:07:39 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> In the context of signal handlers, we may allocate RA memory. That is not ideal but may happen. One example is error reporting - even if we are careful, some code down the stack may use RA. Another example is code running in the context of AsyncGetCallTrace. I'm sure there may be more examples.
>> 
>> The problem is that the signal may (rarely) leave the current thread's RA in an inconsistent state, especially if it got interrupted in the middle of a chunk turnover. Subsequent allocations from it inside the signal handler then would malfunction.
>> 
>> A simple solution would be double buffering. Let each thread have a second resource area, to be used only in signal handling. At the entrance of the hotspot signal handler (which everyone goes through, even in chain scenarios like with AsyncGetCallTrace) we would switch over to the secondary resource area, and switch back when leaving the hotspot signal handler.
>> 
>> Note that I proposed this on hs-runtime-dev [1] but I am actually not sure if the mailing lists work, since I did not see that mail delivered to subscribers. Therefore I went ahead and implemented a simple prototype. 
>> 
>> The prototype keeps matters simple:
>> - we just use two resource areas: the normal one and an alternate one for signal handling. So we don't handle recursive calls to signal handlers, see comment in signals_posix.cpp.
>> - we preallocate both resource area at thread creation time. For the pros and cons of pre-allocating them vs creating them on demand, and possible further improvements, pls see [1].
>> 
>> Tests:
>> - SAP nightlies
>> - GHAs
>> - I tested this manually by corrupting the resource area of a thread, then faulting, and inside the signal handler, I was able to use the secondary resource area as expected.
>> - Automated tests are somewhat more difficult, akin to the existing SafeFetchInErrorHandlerTest. I am not sure if its worth the complexity.
>> 
>> [1] https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2022-February/054126.html
>
> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
> 
>   simplification, copyright update

Hi Thomas,

Seems a reasonable idea in principle, but I'm unsure of some details.

>  At the entrance of the hotspot signal handler (which everyone goes through, even in chain scenarios like with AsyncGetCallTrace) 

That is not my understanding. For AGCT the signal handler is installed directly for, e.g. SIGPROF, and we don't intercept or chain it in the VM AFAIK. Also UserHandler is separate from JVM_HANDLE_XXX_SIGNAL.

Further comments below.

Thanks,
David

src/hotspot/os/posix/signals_posix.cpp line 517:

> 515: 
> 516: // When entering signal handling with a valid current Thread, we switch to the
> 517: //  Thread's secondary resource area. That lets us safely use resource area

Style nit: no need to indent paragraph continuation lines.

src/hotspot/os/posix/signals_posix.cpp line 521:

> 519: //  an inconsistent state. We then swap back to the primary area upon leaving
> 520: //  the signal handler.
> 521: // Note that signal handlers can nest, and in theory we should do this on each

So what happens if you get a secondary signal whilst in the midst of trying to switch to the secondary resource area? Seems we need to ensure signals be deferred whilst this happens so that it is atomic with respect to subsequent signals.

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PR: https://git.openjdk.java.net/jdk/pull/7624