signalfd: retrieve multiple signals with one read() call

Gathering signals in bulk enables server applications to drain a signal
queue (almost full of realtime signals) more efficiently by reducing the
syscall and file look-up overhead.

Very similar to the sigtimedwait4() call described by Niels Provos, Chuck
Lever, and Stephen Tweedie in a paper entitled "Analyzing the Overload
Behavior of a Simple Web Server".  The paper lists more details and
advantages.
Signed-off-by: Davi E. M. Arnaut <davi@haxent.com.br>
Acked-by: Davide Libenzi <davidel@xmailserver.org>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/signalfd.c

@@ -11,6 +11,8 @@
  *      Now using anonymous inode source.
  *      Thanks to Oleg Nesterov for useful code review and suggestions.
  *      More comments and suggestions from Arnd Bergmann.
+ * Sat May 19, 2007: Davi E. M. Arnaut <davi@haxent.com.br>
+ *      Retrieve multiple signals with one read() call
  */
 
 #include <linux/file.h>
@@ -206,64 +208,92 @@ static int signalfd_copyinfo(struct signalfd_siginfo __user *uinfo,
 	return err ? -EFAULT: sizeof(*uinfo);
 }
 
-/*
- * Returns either the size of a "struct signalfd_siginfo", or zero if the
- * sighand we are attached to, has been orphaned. The "count" parameter
- * must be at least the size of a "struct signalfd_siginfo".
- */
-static ssize_t signalfd_read(struct file *file, char __user *buf, size_t count,
-			     loff_t *ppos)
+static ssize_t signalfd_dequeue(struct signalfd_ctx *ctx, siginfo_t *info,
+				int nonblock)
 {
-	struct signalfd_ctx *ctx = file->private_data;
-	ssize_t res = 0;
-	int locked, signo;
-	siginfo_t info;
+	ssize_t ret;
 	struct signalfd_lockctx lk;
 	DECLARE_WAITQUEUE(wait, current);
 
-	if (count < sizeof(struct signalfd_siginfo))
-		return -EINVAL;
-	locked = signalfd_lock(ctx, &lk);
-	if (!locked)
+	if (!signalfd_lock(ctx, &lk))
 		return 0;
-	res = -EAGAIN;
-	signo = dequeue_signal(lk.tsk, &ctx->sigmask, &info);
-	if (signo == 0 && !(file->f_flags & O_NONBLOCK)) {
+
+	ret = dequeue_signal(lk.tsk, &ctx->sigmask, info);
+	switch (ret) {
+	case 0:
+		if (!nonblock)
+			break;
+		ret = -EAGAIN;
+	default:
+		signalfd_unlock(&lk);
+		return ret;
+	}
+
 	add_wait_queue(&ctx->wqh, &wait);
 	for (;;) {
 		set_current_state(TASK_INTERRUPTIBLE);
-			signo = dequeue_signal(lk.tsk, &ctx->sigmask, &info);
-			if (signo != 0)
+		ret = dequeue_signal(lk.tsk, &ctx->sigmask, info);
+		signalfd_unlock(&lk);
+		if (ret != 0)
 			break;
 		if (signal_pending(current)) {
-				res = -ERESTARTSYS;
+			ret = -ERESTARTSYS;
 			break;
 		}
-			signalfd_unlock(&lk);
 		schedule();
-			locked = signalfd_lock(ctx, &lk);
-			if (unlikely(!locked)) {
+		ret = signalfd_lock(ctx, &lk);
+		if (unlikely(!ret)) {
 			/*
 			 * Let the caller read zero byte, ala socket
 			 * recv() when the peer disconnect. This test
 			 * must be done before doing a dequeue_signal(),
 			 * because if the sighand has been orphaned,
-				 * the dequeue_signal() call is going to crash.
+			 * the dequeue_signal() call is going to crash
+			 * because ->sighand will be long gone.
 			 */
-				res = 0;
 			 break;
 		}
 	}
+
 	remove_wait_queue(&ctx->wqh, &wait);
 	__set_current_state(TASK_RUNNING);
-	}
-	if (likely(locked))
-		signalfd_unlock(&lk);
-	if (likely(signo))
-		res = signalfd_copyinfo((struct signalfd_siginfo __user *) buf,
-					&info);
 
-	return res;
+	return ret;
+}
+
+/*
+ * Returns either the size of a "struct signalfd_siginfo", or zero if the
+ * sighand we are attached to, has been orphaned. The "count" parameter
+ * must be at least the size of a "struct signalfd_siginfo".
+ */
+static ssize_t signalfd_read(struct file *file, char __user *buf, size_t count,
+			     loff_t *ppos)
+{
+	struct signalfd_ctx *ctx = file->private_data;
+	struct signalfd_siginfo __user *siginfo;
+	int nonblock = file->f_flags & O_NONBLOCK;
+	ssize_t ret, total = 0;
+	siginfo_t info;
+
+	count /= sizeof(struct signalfd_siginfo);
+	if (!count)
+		return -EINVAL;
+
+	siginfo = (struct signalfd_siginfo __user *) buf;
+
+	do {
+		ret = signalfd_dequeue(ctx, &info, nonblock);
+		if (unlikely(ret <= 0))
+			break;
+		ret = signalfd_copyinfo(siginfo, &info);
+		if (ret < 0)
+			break;
+		siginfo++;
+		total += ret;
+		nonblock = 1;
+	} while (--count);
+
+	return total ? total : ret;
 }
 
 static const struct file_operations signalfd_fops = {