In accept1(), falloc() is called after the process has awoken, but prior

to removing the connection from the queue. The problem here is that
falloc() may block and this would allow another process to accept the
connection instead. If this happens to leave the queue empty, then the
system will panic with an "accept: nothing queued".

Also changed a wakeup() to a wakeup_one() to avoid the "thundering herd"
problem on new connections in Apache (or any other application that has
multiple processes blocked in accept() for the same socket).

1 files changed, 22 insertions, 6 deletions

diff --git a/sys/kern/uipc_syscalls.c b/sys/kern/uipc_syscalls.c
index e3aca30..2db7778 100644
--- a/sys/kern/uipc_syscalls.c
+++ b/sys/kern/uipc_syscalls.c
@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)uipc_syscalls.c	8.4 (Berkeley) 2/21/94
- * $Id: uipc_syscalls.c,v 1.22 1997/02/22 09:39:29 peter Exp $
+ * $Id: uipc_syscalls.c,v 1.23 1997/03/23 03:36:32 bde Exp $
  */
 
 #include "opt_ktrace.h"
@@ -208,20 +208,36 @@ accept1(p, uap, retval, compat)
 		splx(s);
 		return (error);
 	}
+
+	/*
+	 * At this point we know that there is at least one connection
+	 * ready to be accepted. Remove it from the queue prior to
+	 * allocating the file descriptor for it since falloc() may
+	 * block allowing another process to accept the connection
+	 * instead.
+	 */
+	so = head->so_comp.tqh_first;
+	TAILQ_REMOVE(&head->so_comp, so, so_list);
+	head->so_qlen--;
+
 	fflag = fp->f_flag;
 	error = falloc(p, &fp, retval);
 	if (error) {
+		/*
+		 * Probably ran out of file descriptors. Put the
+		 * unaccepted connection back onto the queue and
+		 * do another wakeup so some other process might
+		 * have a chance at it.
+		 */
+		TAILQ_INSERT_HEAD(&head->so_comp, so, so_list);
+		head->so_qlen++;
+		wakeup_one(&head->so_timeo);
 		splx(s);
 		return (error);
 	}
 
-	so = head->so_comp.tqh_first;
-	if (so == NULL)
-		panic("accept: nothing queued");
-	TAILQ_REMOVE(&head->so_comp, so, so_list);
 	so->so_state &= ~SS_COMP;
 	so->so_head = NULL;
-	head->so_qlen--;
 
 	fp->f_type = DTYPE_SOCKET;
 	fp->f_flag = fflag;