Integrated: 8351933: Inaccurate masking of TC subfield decrement in ForkJoinPool

[Dmitry Chuyko]

On Thu, 13 Mar 2025 13:34:51 GMT, Dmitry Chuyko <dchuyko at openjdk.org> wrote:

> Please review a tiny fix in the ForkJoinPool. Since JDK 9 (JDK-8134852 [1]) in one case when TC subfield in ctl field is decremented, the applied masking (UMASK, upper bits) may not preserve neighbor RC subfield sometimes. In JDKs prior to 19 FJP may stop executing tasks, which requires a long running application restart [2]. Since 19 it is even harder to reproduce because of the separate parallelism field.
> 
> The fix is to replace 'UMASK & (c - TC_UNIT)'  with '(c & RC_MASK) | ((c - TC_UNIT)  & TC_MASK)' which preserves the RC part of the compareAndSetCtl() candidate argument. On 17u and 11u that repairs known tests and applications. This PR is for the mainline, and I intend to backport it to 21u, 17u and 11u.
> 
> [1] https://bugs.openjdk.org/browse/JDK-8134852
> [2] https://bugs.openjdk.org/browse/JDK-8330017

This pull request has now been integrated.

Changeset: fed34e46
Author:    Dmitry Chuyko <dchuyko at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/fed34e46b89bc9b0462d9b5f5e5ab5516fe18c6e
Stats:     2 lines in 1 file changed: 1 ins; 0 del; 1 mod

8351933: Inaccurate masking of TC subfield decrement in ForkJoinPool

Reviewed-by: dl, alanb, liach

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PR: https://git.openjdk.org/jdk/pull/24034