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| author | Robert Haas <rhaas@postgresql.org> | 2018-03-05 15:12:49 -0500 |
|---|---|---|
| committer | Robert Haas <rhaas@postgresql.org> | 2018-03-05 15:12:49 -0500 |
| commit | 42d7074ebb832e511d1edec8091cd474d169eef1 (patch) | |
| tree | 8a280a8c71ebe0e82c018230207f8193052925ef | |
| parent | 2f3e2340cd1b67e91cefdf45e4c915297d1034e2 (diff) | |
| download | postgresql-42d7074ebb832e511d1edec8091cd474d169eef1.tar.gz postgresql-42d7074ebb832e511d1edec8091cd474d169eef1.zip | |
shm_mq: Fix detach race condition.
Commit 34db06ef9a1d7f36391c64293bf1e0ce44a33915 adopted a lock-free
design for shm_mq.c, but it introduced a race condition that could
lose messages. When shm_mq_receive_bytes() detects that the other end
has detached, it must make sure that it has seen the final version of
mq_bytes_written, or it might miss a message sent before detaching.
Thomas Munro
Discussion: https://postgr.es/m/CAEepm%3D2myZ4qxpt1a%3DC%2BwEv3o188K13K3UvD-44FK0SdAzHy%2Bw%40mail.gmail.com
| -rw-r--r-- | src/backend/storage/ipc/shm_mq.c | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c index 40df8e1251e..3faace249ff 100644 --- a/src/backend/storage/ipc/shm_mq.c +++ b/src/backend/storage/ipc/shm_mq.c @@ -1058,7 +1058,19 @@ shm_mq_receive_bytes(shm_mq_handle *mqh, Size bytes_needed, bool nowait, * detached. */ if (mq->mq_detached) + { + /* + * If the writer advanced mq_bytes_written and then set + * mq_detached, we might not have read the final value of + * mq_bytes_written above. Insert a read barrier and then check + * again if mq_bytes_written has advanced. + */ + pg_read_barrier(); + if (written != pg_atomic_read_u64(&mq->mq_bytes_written)) + continue; + return SHM_MQ_DETACHED; + } /* * We didn't get enough data to satisfy the request, so mark any data |