sched/membarrier: Introduce MEMBARRIER_CMD_GET_REGISTRATIONS

Provide a method to query previously issued registrations.
Signed-off-by: Michal Clapinski <mclapinski@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20221207164338.1535591-2-mclapinski@google.com

include/uapi/linux/membarrier.h

@@ -137,6 +137,9 @@
  * @MEMBARRIER_CMD_SHARED:
  *                          Alias to MEMBARRIER_CMD_GLOBAL. Provided for
  *                          header backward compatibility.
+ * @MEMBARRIER_CMD_GET_REGISTRATIONS:
+ *                          Returns a bitmask of previously issued
+ *                          registration commands.
  *
  * Command to be passed to the membarrier system call. The commands need to
  * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to
@@ -153,6 +156,7 @@ enum membarrier_cmd {
 	MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE	= (1 << 6),
 	MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ			= (1 << 7),
 	MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ		= (1 << 8),
+	MEMBARRIER_CMD_GET_REGISTRATIONS			= (1 << 9),
 
 	/* Alias for header backward compatibility. */
 	MEMBARRIER_CMD_SHARED			= MEMBARRIER_CMD_GLOBAL,

kernel/sched/membarrier.c

@@ -159,7 +159,8 @@
 	| MEMBARRIER_CMD_PRIVATE_EXPEDITED				\
 	| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED			\
 	| MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK		\
-	| MEMBARRIER_PRIVATE_EXPEDITED_RSEQ_BITMASK)
+	| MEMBARRIER_PRIVATE_EXPEDITED_RSEQ_BITMASK			\
+	| MEMBARRIER_CMD_GET_REGISTRATIONS)
 
 static void ipi_mb(void *info)
 {
@@ -540,6 +541,40 @@ static int membarrier_register_private_expedited(int flags)
 	return 0;
 }
 
+static int membarrier_get_registrations(void)
+{
+	struct task_struct *p = current;
+	struct mm_struct *mm = p->mm;
+	int registrations_mask = 0, membarrier_state, i;
+	static const int states[] = {
+		MEMBARRIER_STATE_GLOBAL_EXPEDITED |
+			MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY,
+		MEMBARRIER_STATE_PRIVATE_EXPEDITED |
+			MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
+		MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE |
+			MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY,
+		MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ |
+			MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY
+	};
+	static const int registration_cmds[] = {
+		MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED,
+		MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED,
+		MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE,
+		MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ
+	};
+	BUILD_BUG_ON(ARRAY_SIZE(states) != ARRAY_SIZE(registration_cmds));
+
+	membarrier_state = atomic_read(&mm->membarrier_state);
+	for (i = 0; i < ARRAY_SIZE(states); ++i) {
+		if (membarrier_state & states[i]) {
+			registrations_mask |= registration_cmds[i];
+			membarrier_state &= ~states[i];
+		}
+	}
+	WARN_ON_ONCE(membarrier_state != 0);
+	return registrations_mask;
+}
+
 /**
  * sys_membarrier - issue memory barriers on a set of threads
  * @cmd:    Takes command values defined in enum membarrier_cmd.
@@ -623,6 +658,8 @@ SYSCALL_DEFINE3(membarrier, int, cmd, unsigned int, flags, int, cpu_id)
 		return membarrier_private_expedited(MEMBARRIER_FLAG_RSEQ, cpu_id);
 	case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ:
 		return membarrier_register_private_expedited(MEMBARRIER_FLAG_RSEQ);
+	case MEMBARRIER_CMD_GET_REGISTRATIONS:
+		return membarrier_get_registrations();
 	default:
 		return -EINVAL;
 	}