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From: Attilio Rao <attilio@FreeBSD.org>
Date: Fri, 6 Nov 2009 22:29:46 +0000 (UTC)
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Subject: svn commit: r199007 - head/sys/fs/fifofs
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Author: attilio
Date: Fri Nov  6 22:29:46 2009
New Revision: 199007
URL: http://svn.freebsd.org/changeset/base/199007

Log:
  - Improve comments about locking of the "struct fifoinfo" which is a bit
    unclear.
  - Fix a memory leak [0]
  
  [0] Diagnosed by:	Dorr H. Clark <dclark at engr dot scu dot edu>
  MFC:	1 week

Modified:
  head/sys/fs/fifofs/fifo_vnops.c

Modified: head/sys/fs/fifofs/fifo_vnops.c
==============================================================================
--- head/sys/fs/fifofs/fifo_vnops.c	Fri Nov  6 22:13:39 2009	(r199006)
+++ head/sys/fs/fifofs/fifo_vnops.c	Fri Nov  6 22:29:46 2009	(r199007)
@@ -78,6 +78,10 @@ struct fileops fifo_ops_f = {
 /*
  * This structure is associated with the FIFO vnode and stores
  * the state associated with the FIFO.
+ * Notes about locking:
+ *   - fi_readsock and fi_writesock are invariant since init time.
+ *   - fi_readers and fi_writers are vnode lock protected.
+ *   - fi_wgen is fif_mtx lock protected.
  */
 struct fifoinfo {
 	struct socket	*fi_readsock;
@@ -223,14 +227,9 @@ fail1:
 	}
 
 	/*
-	 * General access to fi_readers and fi_writers is protected using
-	 * the vnode lock.
-	 *
-	 * Protect the increment of fi_readers and fi_writers and the
-	 * associated calls to wakeup() with the fifo mutex in addition
-	 * to the vnode lock.  This allows the vnode lock to be dropped
-	 * for the msleep() calls below, and using the fifo mutex with
-	 * msleep() prevents the wakeup from being missed.
+	 * Use the fifo_mtx lock here, in addition to the vnode lock,
+	 * in order to allow vnode lock dropping before msleep() calls
+	 * and still avoiding missed wakeups.
 	 */
 	mtx_lock(&fifo_mtx);
 	if (ap->a_mode & FREAD) {
@@ -249,6 +248,8 @@ fail1:
 	if (ap->a_mode & FWRITE) {
 		if ((ap->a_mode & O_NONBLOCK) && fip->fi_readers == 0) {
 			mtx_unlock(&fifo_mtx);
+			if (fip->fi_writers == 0)
+				fifo_cleanup(vp);
 			return (ENXIO);
 		}
 		fip->fi_writers++;