Integrated: 8351997: AArch64: Interpreter volatile reference stores with G1 are not sequentially consistent

[Erik Österlund]

On Wed, 28 May 2025 08:49:17 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

> The optimized fast_aputfield bytecode on AArch64 stores the field flags in r3, and performs the leading and trailing fencing depending on its volatile bit being set or not. However, r3 is also the last temp register passed in to the barrier set for reference stores, and G1 clobbers it in a way that may clear the volatile bit. Then the trailing fence won't get executed, and sequential consistency is broken.
> 
> My fix puts the flags in r5 instead, which is the register that was used by normal aputfield bytecodes. This way, barriers don't clobber the volatile bits.
> 
> This bug has been observed to mess up a classic Dekker duality in the java.util.concurrent.Exchanger class, leading to a hang in the test/jdk/java/util/concurrent/Exchanger/ExchangeLoops.java test that exercises it. Using G1 and -Xint a reproducer hangs 30/100 times in mach5. With the fix, the same reproducer hangs 0/100 times.

This pull request has now been integrated.

Changeset: 83b15da2
Author:    Erik Österlund <eosterlund at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/83b15da2eb3cb6c8937f517c9b75eaa9eeece314
Stats:     8 lines in 1 file changed: 4 ins; 0 del; 4 mod

8351997: AArch64: Interpreter volatile reference stores with G1 are not sequentially consistent

Reviewed-by: shade, aph, fbredberg

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PR: https://git.openjdk.org/jdk/pull/25483