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Date:      Mon, 16 Sep 2019 21:32:21 +0000 (UTC)
From:      Mateusz Guzik <mjg@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r352425 - head/sys/kern
Message-ID:  <201909162132.x8GLWLRu003393@repo.freebsd.org>
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Author: mjg
Date: Mon Sep 16 21:32:21 2019
New Revision: 352425
URL: https://svnweb.freebsd.org/changeset/base/352425

Log:
  vfs: manage mnt_lockref with atomics
  
  See r352424.
  
  Reviewed by:	kib, jeff
  Tested by:	pho
  Sponsored by:	The FreeBSD Foundation
  Differential Revision:	https://reviews.freebsd.org/D21574

Modified:
  head/sys/kern/vfs_subr.c

Modified: head/sys/kern/vfs_subr.c
==============================================================================
--- head/sys/kern/vfs_subr.c	Mon Sep 16 21:31:02 2019	(r352424)
+++ head/sys/kern/vfs_subr.c	Mon Sep 16 21:32:21 2019	(r352425)
@@ -641,6 +641,18 @@ vfs_busy(struct mount *mp, int flags)
 	MPASS((flags & ~MBF_MASK) == 0);
 	CTR3(KTR_VFS, "%s: mp %p with flags %d", __func__, mp, flags);
 
+	if (vfs_op_thread_enter(mp)) {
+		MPASS((mp->mnt_kern_flag & MNTK_DRAINING) == 0);
+		MPASS((mp->mnt_kern_flag & MNTK_UNMOUNT) == 0);
+		MPASS((mp->mnt_kern_flag & MNTK_REFEXPIRE) == 0);
+		MNT_REF_UNLOCKED(mp);
+		atomic_add_int(&mp->mnt_lockref, 1);
+		vfs_op_thread_exit(mp);
+		if (flags & MBF_MNTLSTLOCK)
+			mtx_unlock(&mountlist_mtx);
+		return (0);
+	}
+
 	MNT_ILOCK(mp);
 	MNT_REF(mp);
 	/*
@@ -673,7 +685,7 @@ vfs_busy(struct mount *mp, int flags)
 	}
 	if (flags & MBF_MNTLSTLOCK)
 		mtx_unlock(&mountlist_mtx);
-	mp->mnt_lockref++;
+	atomic_add_int(&mp->mnt_lockref, 1);
 	MNT_IUNLOCK(mp);
 	return (0);
 }
@@ -684,13 +696,24 @@ vfs_busy(struct mount *mp, int flags)
 void
 vfs_unbusy(struct mount *mp)
 {
+	int c;
 
 	CTR2(KTR_VFS, "%s: mp %p", __func__, mp);
+
+	if (vfs_op_thread_enter(mp)) {
+		MPASS((mp->mnt_kern_flag & MNTK_DRAINING) == 0);
+		c = atomic_fetchadd_int(&mp->mnt_lockref, -1) - 1;
+		KASSERT(c >= 0, ("%s: negative mnt_lockref %d\n", __func__, c));
+		MNT_REL_UNLOCKED(mp);
+		vfs_op_thread_exit(mp);
+		return;
+	}
+
 	MNT_ILOCK(mp);
 	MNT_REL(mp);
-	KASSERT(mp->mnt_lockref > 0, ("negative mnt_lockref"));
-	mp->mnt_lockref--;
-	if (mp->mnt_lockref == 0 && (mp->mnt_kern_flag & MNTK_DRAINING) != 0) {
+	c = atomic_fetchadd_int(&mp->mnt_lockref, -1) - 1;
+	KASSERT(c >= 0, ("%s: negative mnt_lockref %d\n", __func__, c));
+	if (c == 0 && (mp->mnt_kern_flag & MNTK_DRAINING) != 0) {
 		MPASS(mp->mnt_kern_flag & MNTK_UNMOUNT);
 		CTR1(KTR_VFS, "%s: waking up waiters", __func__);
 		mp->mnt_kern_flag &= ~MNTK_DRAINING;

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