Add memory barriers for PgBackendStatus.st_changecount protocol

Enterprise / PostgreSQL - Fujii Masao [postgresql.org] - 18 December 2014 08:07 UTC

st_changecount protocol needs the memory barriers to ensure that the apparent order of execution is as it desires. Otherwise, for example, the CPU might rearrange the code so that st_changecount is incremented twice before the modification on a machine with weak memory ordering. This surprising result can lead to bugs.

This commit introduces the macros to load and store st_changecount with the memory barriers. These are called before and after PgBackendStatus entries are modified or copied into private memory, in order to prevent CPU from reordering PgBackendStatus access.

Per discussion on pgsql-hackers, we decided not to back-patch this to 9.4 or before until we get an actual bug report about this.

Patch by me. Review by Robert Haas.

38628db Add memory barriers for PgBackendStatus.st_changecount protocol.
src/backend/postmaster/pgstat.c | 51 +++++++++++++++++++++------------------
src/include/pgstat.h | 44 +++++++++++++++++++++++++++++++++
2 files changed, 71 insertions(+), 24 deletions(-)

Upstream: git.postgresql.org

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