Fix race between ProcSignalInit() and EmitProcSignalBarrier().

Previously, ProcSignalInit() read the global barrier generation before
publishing its PID into pss_pid. This created a race condition: a
process could initialize its local generation with an older global
value, while a concurrent EmitProcSignalBarrier() might skip that
process because its pss_pid was still zero. This resulted in
WaitForProcSignalBarrier() hanging indefinitely.

Fix this by publishing pss_pid before reading psh_barrierGeneration
with a memory barrier so that the store to pss_pid is ordered before
the load. A concurrent EmitProcSignalBarrier() then either observes
the published PID and signals this slot, or completes its generation
increment before we load it.

While this race has become more visible due to recent features using
signal barriers in more places (such as online wal_level changes), the
issue is theoretically present since signal barriers were introduced
to release smgr caches (e.g., in DROP DATABASE). v14 has the
procsiangl barrier infrastricutre but no in-tree caller that actually
emits a barrier, so the case is unreachable there.

This issue was also reported by buildfarm member flaviventris.

Reported-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/CAEze2WgAJmWReDN7Chtba8Er2YBvKCoa0KVN25-1evnTrHsLyA@mail.gmail.com
Backpatch-through: 15

diff --git a/src/backend/storage/ipc/procsignal.c b/src/backend/storage/ipc/procsignal.c
index c85cb5cc18dbb9875751517303a744d9fce7dcf6..9dfe000353d41d709135486768d408a17c030c78 100644 (file)
--- a/src/backend/storage/ipc/procsignal.c
+++ b/src/backend/storage/ipc/procsignal.c
@@ -176,6 +176,16 @@ ProcSignalInit(int pss_idx)
        /* Clear out any leftover signal reasons */
        MemSet(slot->pss_signalFlags, 0, NUM_PROCSIGNALS * sizeof(sig_atomic_t));
 
+       /*
+        * Publish the PID before reading the global barrier generation to ensure
+        * that EmitProcSignalBarrier() doesn't skip us while we are grabbing an
+        * older generation. We need a memory barrier here to make sure that the
+        * update of pss_pid is ordered before the subsequent load of
+        * psh_barrierGeneration.
+        */
+       slot->pss_pid = MyProcPid;
+       pg_memory_barrier();
+
        /*
         * Initialize barrier state. Since we're a brand-new process, there
         * shouldn't be any leftover backend-private state that needs to be
@@ -193,9 +203,6 @@ ProcSignalInit(int pss_idx)
        pg_atomic_write_u64(&slot->pss_barrierGeneration, barrier_generation);
        pg_memory_barrier();
 
-       /* Mark slot with my PID */
-       slot->pss_pid = MyProcPid;
-
        /* Remember slot location for CheckProcSignal */
        MyProcSignalSlot = slot;