> The fix is to set bit 13 of MCSR24 to 1. MCSR24 is an internal CSR whose function is not described in the manual. After setting this bit, the lost update no longer occurs. Testing showed the performance loss is very small: single-core performance is unaffected, and multi-core performance drops only slightly.
Presumably this is a so-called “chicken bit”.
Some optimization that turned out to be buggy had a disable bit in an undocumented register.
For those who are out of the loop:
> https://lwn.net/Articles/744840/
See also
> https://en.wiktionary.org/wiki/chicken_bit
(click on "quotations") for a list of scientific texts in which this term occurs.