evl/monitor: event: disable syscall restart on interrupted wait

When interrupted by a signal, a wait operation on a gated monitor
should not be restarted automatically from kernel space, but reported
to the user instead.

Rationale: the mutex guarding the monitor was dropped prior to
waiting, and must be re-acquired explicitly by an UNWAIT request when
the interrupted status is detected from user-space. Consequently, we
cannot retry a WAIT request without transitioning to user-space first.

Add the T_NORST local information bit to the thread control block,
which is set by the monitor wait operation, and tested by the syscall
trampoline code for disabling automatic restart.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>

include/uapi/evl/thread.h

@@ -62,6 +62,7 @@
 #define T_SYSRST  0x00000001 /* Thread awaiting syscall restart after signal */
 #define T_IGNOVR  0x00000002 /* Overrun detection temporarily disabled */
 #define T_INFAULT 0x00000004 /* In fault handling */
+#define T_NORST   0x00000008 /* Disable syscall restart */
 
 /*
  * Must follow strictly the declaration order of the state flags

kernel/evl/monitor.c

@@ -527,13 +527,19 @@ static int wait_monitor(struct file *filp,
 	 * be handled asap. So exit to user mode, allowing any pending
 	 * signal to be handled during the transition, then expect
 	 * userland to issue UNWAIT to recover (or exit, whichever
-	 * comes first).
+	 * comes first). Consequently, disable syscall restart from
+	 * kernel upon interrupted wait, because the caller does not
+	 * hold the mutex until UNWAIT happens.
 	 */
 	ret = evl_wait_schedule(&event->wait_queue);
 	if (ret) {
 		untrack_event(event);
-		if (ret == -EINTR)
+		if (ret == -EINTR) {
+			/* Disable syscall restart upon signal (only). */
+			if (signal_pending(current))
+				curr->local_info |= T_NORST;
 			goto put;
+		}
 		op_ret = ret;
 	}
 

kernel/evl/syscall.c

@@ -164,13 +164,6 @@ static void prepare_for_signal(struct task_struct *p,
 {
 	unsigned long flags;
 
-	/*
-	 * FIXME: no restart mode flag for setting -EINTR instead of
-	 * -ERESTARTSYS should be obtained from curr->local_info on a
-	 * per-invocation basis, not on a per-call one (since we have
-	 * 3 generic calls only).
-	 */
-
 	/*
 	 * @curr == this_evl_rq()->curr over oob so no need to grab
 	 * @curr->lock (i.e. @curr cannot go away under out feet).
@@ -187,7 +180,12 @@ static void prepare_for_signal(struct task_struct *p,
 	 */
 	if (curr->info & T_KICKED) {
 		if (signal_pending(p)) {
-			syscall_set_return_value(current, regs, -ERESTARTSYS, 0);
+			int retval = -ERESTARTSYS;
+			if (curr->local_info & T_NORST) {
+				retval = -EINTR;
+				curr->local_info &= ~T_NORST;
+			}
+			syscall_set_return_value(current, regs, retval, 0);
 			curr->info &= ~T_BREAK;
 		}
 		curr->info &= ~T_KICKED;