[PATCH 2/3] signals: introduce SIGQUEUE_CANCELLED

[Oleg Nesterov]

Suggested by Linus Torvalds.

Introduce SIGQUEUE_CANCELLED flag. If it is set, the pending signal should be
ignored and collect_signal() should return 0.

Note that the new flag is completely orthogonal to SIGQUEUE_PREALLOC, and it
can be set without ->siglock held.

No users of this new flag yet, see the next patch.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

--- 25/include/linux/signal.h~5_INTRODUCE_CANCELLED	2008-03-04 21:43:25.000000000 +0300
+++ 25/include/linux/signal.h	2008-03-04 21:43:25.000000000 +0300
@@ -20,6 +20,7 @@ struct sigqueue {
 
 /* flags values. */
 #define SIGQUEUE_PREALLOC	1
+#define SIGQUEUE_CANCELLED	2
 
 struct sigpending {
 	struct list_head list;
--- 25/kernel/signal.c~5_INTRODUCE_CANCELLED	2008-05-17 17:14:04.000000000 +0400
+++ 25/kernel/signal.c	2008-05-17 17:40:09.000000000 +0400
@@ -326,11 +326,13 @@ static int collect_signal(int sig, struc
 		}
 	}
 	if (first) {
-		list_del_init(&first->list);
-		copy_siginfo(info, &first->info);
-		__sigqueue_free(first);
 		if (!still_pending)
 			sigdelset(&list->signal, sig);
+		if (unlikely(first->flags & SIGQUEUE_CANCELLED))
+			sig = 0;
+		copy_siginfo(info, &first->info);
+		list_del_init(&first->list);
+		__sigqueue_free(first);
 	} else {
 
 		/* Ok, it wasn't in the queue.  This must be
@@ -344,7 +346,7 @@ static int collect_signal(int sig, struc
 		info->si_pid = 0;
 		info->si_uid = 0;
 	}
-	return 1;
+	return sig;
 }
 
 static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
@@ -362,8 +364,7 @@ static int __dequeue_signal(struct sigpe
 			}
 		}
 
-		if (!collect_signal(sig, pending, info))
-			sig = 0;
+		sig = collect_signal(sig, pending, info);
 	}
 
 	return sig;

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