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Use barrier semantics when reading/writing writtenUpto

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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 <andres@anarazel.de>
Discussion: https://postgr.es/m/zqbppucpmkeqecfy4s5kscnru4tbk6khp3ozqz6ad2zijz354k%40w4bdf4z3wqoz
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
This commit is contained in:
Alexander Korotkov 2026-05-03 16:21:25 +03:00
parent c34a280c85
commit dfb690dd52
2 changed files with 12 additions and 4 deletions
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2
src/backend/replication/walreceiver.c
View file

@ -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.

14
src/backend/replication/walreceiverfuncs.c
View file

@ -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);
}
/*