Avoid potential spinlock in a signal handler as part of global barriers.

On platforms without support for 64bit atomic operations where we also
cannot rely on 64bit reads to have single copy atomicity, such atomics
are implemented using a spinlock based fallback. That means it's not
safe to even read such atomics from within a signal handler (since the
signal handler might run when the spinlock already is held).

To avoid this issue defer global barrier processing out of the signal
handler. Instead of checking local / shared barrier generation to
determine whether to set ProcSignalBarrierPending, introduce
PROCSIGNAL_BARRIER and always set ProcSignalBarrierPending when
receiving such a signal. Additionally avoid redundant work in
ProcessProcSignalBarrier if ProcSignalBarrierPending is unnecessarily.

Also do a small amount of other polishing.

Author: Andres Freund
Reviewed-By: Robert Haas
Discussion: https://postgr.es/m/20200609193723.eu5ilsjxwdpyxhgz@alap3.anarazel.de
Backpatch: 13-, where the code was introduced.

src/backend/storage/ipc/procsignal.c

@@ -320,7 +320,7 @@ SendProcSignal(pid_t pid, ProcSignalReason reason, BackendId backendId)
 uint64
 EmitProcSignalBarrier(ProcSignalBarrierType type)
 {
-	uint64		flagbit = UINT64CONST(1) << (uint64) type;
+	uint32		flagbit = 1 << (uint32) type;
 	uint64		generation;
 
 	/*
@@ -363,7 +363,11 @@ EmitProcSignalBarrier(ProcSignalBarrierType type)
 		pid_t		pid = slot->pss_pid;
 
 		if (pid != 0)
+		{
+			/* see SendProcSignal for details */
+			slot->pss_signalFlags[PROCSIG_BARRIER] = true;
 			kill(pid, SIGUSR1);
+		}
 	}
 
 	return generation;
@@ -383,6 +387,8 @@ WaitForProcSignalBarrier(uint64 generation)
 {
 	long		timeout = 125L;
 
+	Assert(generation <= pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration));
+
 	for (int i = NumProcSignalSlots - 1; i >= 0; i--)
 	{
 		volatile ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
@@ -417,6 +423,23 @@ WaitForProcSignalBarrier(uint64 generation)
 	pg_memory_barrier();
 }
 
+/*
+ * Handle receipt of an interrupt indicating a global barrier event.
+ *
+ * All the actual work is deferred to ProcessProcSignalBarrier(), because we
+ * cannot safely access the barrier generation inside the signal handler as
+ * 64bit atomics might use spinlock based emulation, even for reads. As this
+ * routine only gets called when PROCSIG_BARRIER is sent that won't cause a
+ * lot fo unnecessary work.
+ */
+static void
+HandleProcSignalBarrierInterrupt(void)
+{
+	InterruptPending = true;
+	ProcSignalBarrierPending = true;
+	/* latch will be set by procsignal_sigusr1_handler */
+}
+
 /*
  * Perform global barrier related interrupt checking.
  *
@@ -428,22 +451,38 @@ WaitForProcSignalBarrier(uint64 generation)
 void
 ProcessProcSignalBarrier(void)
 {
-	uint64		generation;
+	uint64		local_gen;
+	uint64		shared_gen;
 	uint32		flags;
 
+	Assert(MyProcSignalSlot);
+
 	/* Exit quickly if there's no work to do. */
 	if (!ProcSignalBarrierPending)
 		return;
 	ProcSignalBarrierPending = false;
 
 	/*
-	 * Read the current barrier generation, and then get the flags that are
+	 * It's not unlikely to process multiple barriers at once, before the
-	 * set for this backend. Note that pg_atomic_exchange_u32 is a full
+	 * signals for all the barriers have arrived. To avoid unnecessary work in
-	 * barrier, so we're guaranteed that the read of the barrier generation
+	 * response to subsequent signals, exit early if we already have processed
-	 * happens before we atomically extract the flags, and that any subsequent
+	 * all of them.
-	 * state changes happen afterward.
+	 */
+	local_gen = pg_atomic_read_u64(&MyProcSignalSlot->pss_barrierGeneration);
+	shared_gen = pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration);
+
+	Assert(local_gen <= shared_gen);
+
+	if (local_gen == shared_gen)
+		return;
+
+	/*
+	 * Get and clear the flags that are set for this backend. Note that
+	 * pg_atomic_exchange_u32 is a full barrier, so we're guaranteed that the
+	 * read of the barrier generation above happens before we atomically
+	 * extract the flags, and that any subsequent state changes happen
+	 * afterward.
 	 */
-	generation = pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration);
 	flags = pg_atomic_exchange_u32(&MyProcSignalSlot->pss_barrierCheckMask, 0);
 
 	/*
@@ -466,7 +505,7 @@ ProcessProcSignalBarrier(void)
 	 * things have changed further, it'll get fixed up when this function is
 	 * next called.
 	 */
-	pg_atomic_write_u64(&MyProcSignalSlot->pss_barrierGeneration, generation);
+	pg_atomic_write_u64(&MyProcSignalSlot->pss_barrierGeneration, shared_gen);
 }
 
 static void
@@ -505,27 +544,6 @@ CheckProcSignal(ProcSignalReason reason)
 	return false;
 }
 
-/*
- * CheckProcSignalBarrier - check for new barriers we need to absorb
- */
-static bool
-CheckProcSignalBarrier(void)
-{
-	volatile ProcSignalSlot *slot = MyProcSignalSlot;
-
-	if (slot != NULL)
-	{
-		uint64		mygen;
-		uint64		curgen;
-
-		mygen = pg_atomic_read_u64(&slot->pss_barrierGeneration);
-		curgen = pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration);
-		return (mygen != curgen);
-	}
-
-	return false;
-}
-
 /*
  * procsignal_sigusr1_handler - handle SIGUSR1 signal.
  */
@@ -546,6 +564,9 @@ procsignal_sigusr1_handler(SIGNAL_ARGS)
 	if (CheckProcSignal(PROCSIG_WALSND_INIT_STOPPING))
 		HandleWalSndInitStopping();
 
+	if (CheckProcSignal(PROCSIG_BARRIER))
+		HandleProcSignalBarrierInterrupt();
+
 	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_DATABASE))
 		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_DATABASE);
 
@@ -564,12 +585,6 @@ procsignal_sigusr1_handler(SIGNAL_ARGS)
 	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN))
 		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN);
 
-	if (CheckProcSignalBarrier())
-	{
-		InterruptPending = true;
-		ProcSignalBarrierPending = true;
-	}
-
 	SetLatch(MyLatch);
 
 	latch_sigusr1_handler();

src/include/storage/procsignal.h

@@ -33,6 +33,7 @@ typedef enum
 	PROCSIG_NOTIFY_INTERRUPT,	/* listen/notify interrupt */
 	PROCSIG_PARALLEL_MESSAGE,	/* message from cooperating parallel backend */
 	PROCSIG_WALSND_INIT_STOPPING,	/* ask walsenders to prepare for shutdown  */
+	PROCSIG_BARRIER,			/* global barrier interrupt  */
 
 	/* Recovery conflict reasons */
 	PROCSIG_RECOVERY_CONFLICT_DATABASE,