From: Alexander Korotkov Date: Sun, 3 May 2026 13:21:25 +0000 (+0300) Subject: Use barrier semantics when reading/writing writtenUpto X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=dfb690dd52375517138ce1ee0f20a831d9585778;p=thirdparty%2Fpostgresql.git Use barrier semantics when reading/writing writtenUpto The walreceiver publishes its write position lock-free via writtenUpto. On weakly-ordered architectures (ARM, PowerPC), both sides of this handshake need explicit barriers so that the lock-less reader sees a consistent state. Use pg_atomic_write_membarrier_u64() at both write sites and pg_atomic_read_membarrier_u64() in GetWalRcvWriteRecPtr(). This matches the barrier semantics that GetWalRcvFlushRecPtr() and other LSN-position functions get implicitly from their spinlock acquire/release, and protects from bugs caused by expectations of similar barrier guarantees from different LSN-position functions. Reported-by: Andres Freund Discussion: https://postgr.es/m/zqbppucpmkeqecfy4s5kscnru4tbk6khp3ozqz6ad2zijz354k%40w4bdf4z3wqoz Author: Xuneng Zhou Reviewed-by: Andres Freund Reviewed-by: Alexander Korotkov --- diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c index df03dae5836..9e40f066c97 100644 --- a/src/backend/replication/walreceiver.c +++ b/src/backend/replication/walreceiver.c @@ -978,7 +978,7 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli) } /* Update shared-memory status */ - pg_atomic_write_u64(&WalRcv->writtenUpto, LogstreamResult.Write); + pg_atomic_write_membarrier_u64(&WalRcv->writtenUpto, LogstreamResult.Write); /* * If we wrote an LSN that someone was waiting for, notify the waiters. diff --git a/src/backend/replication/walreceiverfuncs.c b/src/backend/replication/walreceiverfuncs.c index bd5d47be964..a0ed853e2f6 100644 --- a/src/backend/replication/walreceiverfuncs.c +++ b/src/backend/replication/walreceiverfuncs.c @@ -321,7 +321,12 @@ RequestXLogStreaming(TimeLineID tli, XLogRecPtr recptr, const char *conninfo, walrcv->flushedUpto = recptr; walrcv->receivedTLI = tli; walrcv->latestChunkStart = recptr; - pg_atomic_write_u64(&walrcv->writtenUpto, recptr); + + /* + * Pairs with pg_atomic_read_membarrier_u64() in + * GetWalRcvWriteRecPtr(). + */ + pg_atomic_write_membarrier_u64(&walrcv->writtenUpto, recptr); } walrcv->receiveStart = recptr; walrcv->receiveStartTLI = tli; @@ -363,14 +368,17 @@ GetWalRcvFlushRecPtr(XLogRecPtr *latestChunkStart, TimeLineID *receiveTLI) /* * Returns the last+1 byte position that walreceiver has written. - * This returns a recently written value without taking a lock. + * + * Use pg_atomic_read_membarrier_u64() to ensure that callers see up-to-date + * shared memory state, matching the barrier semantics provided by the + * spinlock in GetWalRcvFlushRecPtr() and other LSN-position functions. */ XLogRecPtr GetWalRcvWriteRecPtr(void) { WalRcvData *walrcv = WalRcv; - return pg_atomic_read_u64(&walrcv->writtenUpto); + return pg_atomic_read_membarrier_u64(&walrcv->writtenUpto); } /*