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Date:      Wed, 2 Nov 2016 09:43:19 +0000 (UTC)
From:      Edward Tomasz Napierala <trasz@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r308209 - head/sys/kern
Message-ID:  <201611020943.uA29hJda033034@repo.freebsd.org>
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Author: trasz
Date: Wed Nov  2 09:43:19 2016
New Revision: 308209
URL: https://svnweb.freebsd.org/changeset/base/308209

Log:
  Fix getfsstat(2) with MNT_WAIT to not skip filesystems that are in the
  process of being unmounted.  Previously it would skip them, even if the
  unmount eventually failed eg due to the filesystem being busy.
  
  This behaviour broke autounmountd(8) - if you tried to manually unmount
  a mounted filesystem, using 'automount -u', and the autounmountd attempted
  to refresh the filesystem list in that very moment, it would conclude that
  the filesystem got unmounted and not try to unmount it afterwards.
  
  Reviewed by:	kib@
  Tested by:	pho@
  MFC after:	1 month
  Differential Revision:	https://reviews.freebsd.org/D8030

Modified:
  head/sys/kern/vfs_syscalls.c

Modified: head/sys/kern/vfs_syscalls.c
==============================================================================
--- head/sys/kern/vfs_syscalls.c	Wed Nov  2 08:46:04 2016	(r308208)
+++ head/sys/kern/vfs_syscalls.c	Wed Nov  2 09:43:19 2016	(r308209)
@@ -446,16 +446,19 @@ kern_getfsstat(struct thread *td, struct
     size_t *countp, enum uio_seg bufseg, int flags)
 {
 	struct mount *mp, *nmp;
-	struct statfs *sfsp, *sp, sb;
+	struct statfs *sfsp, *sp, sb, *tofree;
 	size_t count, maxcount;
 	int error;
 
+restart:
 	maxcount = bufsize / sizeof(struct statfs);
-	if (bufsize == 0)
+	if (bufsize == 0) {
 		sfsp = NULL;
-	else if (bufseg == UIO_USERSPACE)
+		tofree = NULL;
+	} else if (bufseg == UIO_USERSPACE) {
 		sfsp = *buf;
-	else /* if (bufseg == UIO_SYSSPACE) */ {
+		tofree = NULL;
+	} else /* if (bufseg == UIO_SYSSPACE) */ {
 		count = 0;
 		mtx_lock(&mountlist_mtx);
 		TAILQ_FOREACH(mp, &mountlist, mnt_list) {
@@ -464,8 +467,8 @@ kern_getfsstat(struct thread *td, struct
 		mtx_unlock(&mountlist_mtx);
 		if (maxcount > count)
 			maxcount = count;
-		sfsp = *buf = malloc(maxcount * sizeof(struct statfs), M_TEMP,
-		    M_WAITOK);
+		tofree = sfsp = *buf = malloc(maxcount * sizeof(struct statfs),
+		    M_TEMP, M_WAITOK);
 	}
 	count = 0;
 	mtx_lock(&mountlist_mtx);
@@ -480,9 +483,24 @@ kern_getfsstat(struct thread *td, struct
 			continue;
 		}
 #endif
-		if (vfs_busy(mp, MBF_NOWAIT | MBF_MNTLSTLOCK)) {
-			nmp = TAILQ_NEXT(mp, mnt_list);
-			continue;
+		if (flags == MNT_WAIT) {
+			if (vfs_busy(mp, MBF_MNTLSTLOCK) != 0) {
+				/*
+				 * If vfs_busy() failed, and MBF_NOWAIT
+				 * wasn't passed, then the mp is gone.
+				 * Furthermore, because of MBF_MNTLSTLOCK,
+				 * the mountlist_mtx was dropped.  We have
+				 * no other choice than to start over.
+				 */
+				mtx_unlock(&mountlist_mtx);
+				free(tofree, M_TEMP);
+				goto restart;
+			}
+		} else {
+			if (vfs_busy(mp, MBF_NOWAIT | MBF_MNTLSTLOCK) != 0) {
+				nmp = TAILQ_NEXT(mp, mnt_list);
+				continue;
+			}
 		}
 		if (sfsp && count < maxcount) {
 			sp = &mp->mnt_stat;

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