From a9b92aa58d7925328530f380fc9e347591ccba30 Mon Sep 17 00:00:00 2001
From: kib <kib@FreeBSD.org>
Date: Sat, 21 May 2016 11:40:41 +0000
Subject: [PATCH] Same as for UFS, remove drop/reacquire of Giant, and use
 si_mountpt as the mount semaphore.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
---
 sys/fs/msdosfs/msdosfs_vfsops.c | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/sys/fs/msdosfs/msdosfs_vfsops.c b/sys/fs/msdosfs/msdosfs_vfsops.c
index d4b6a025f65b..63d60dde015e 100644
--- a/sys/fs/msdosfs/msdosfs_vfsops.c
+++ b/sys/fs/msdosfs/msdosfs_vfsops.c
@@ -292,11 +292,9 @@ msdosfs_mount(struct mount *mp)
 			}
 
 			/* Downgrade the device from rw to ro. */
-			DROP_GIANT();
 			g_topology_lock();
 			error = g_access(pmp->pm_cp, 0, -1, 0);
 			g_topology_unlock();
-			PICKUP_GIANT();
 			if (error) {
 				(void)markvoldirty(pmp, 1);
 				return (error);
@@ -328,11 +326,9 @@ msdosfs_mount(struct mount *mp)
 				return (error);
 			}
 			VOP_UNLOCK(devvp, 0);
-			DROP_GIANT();
 			g_topology_lock();
 			error = g_access(pmp->pm_cp, 0, 1, 0);
 			g_topology_unlock();
-			PICKUP_GIANT();
 			if (error)
 				return (error);
 
@@ -401,8 +397,6 @@ msdosfs_mount(struct mount *mp)
 		return error;
 	}
 
-	if (devvp->v_type == VCHR && devvp->v_rdev != NULL)
-		devvp->v_rdev->si_mountpt = mp;
 	vfs_mountedfrom(mp, from);
 #ifdef MSDOSFS_DEBUG
 	printf("msdosfs_mount(): mp %p, pmp %p, inusemap %p\n", mp, pmp, pmp->pm_inusemap);
@@ -431,15 +425,21 @@ mountmsdosfs(struct vnode *devvp, struct mount *mp)
 	ronly = (mp->mnt_flag & MNT_RDONLY) != 0;
 
 	dev = devvp->v_rdev;
-	dev_ref(dev);
-	DROP_GIANT();
+	if (atomic_cmpset_acq_ptr((uintptr_t *)&dev->si_mountpt, 0,
+	    (uintptr_t)mp) == 0) {
+		VOP_UNLOCK(devvp, 0);
+		return (EBUSY);
+	}
 	g_topology_lock();
 	error = g_vfs_open(devvp, &cp, "msdosfs", ronly ? 0 : 1);
 	g_topology_unlock();
-	PICKUP_GIANT();
+	if (error != 0) {
+		atomic_store_rel_ptr((uintptr_t *)&dev->si_mountpt, 0);
+		VOP_UNLOCK(devvp, 0);
+		return (error);
+	}
+	dev_ref(dev);
 	VOP_UNLOCK(devvp, 0);
-	if (error)
-		goto error_exit;
 
 	bo = &devvp->v_bufobj;
 
@@ -770,11 +770,9 @@ error_exit:
 	if (bp)
 		brelse(bp);
 	if (cp != NULL) {
-		DROP_GIANT();
 		g_topology_lock();
 		g_vfs_close(cp);
 		g_topology_unlock();
-		PICKUP_GIANT();
 	}
 	if (pmp) {
 		lockdestroy(&pmp->pm_fatlock);
@@ -783,6 +781,7 @@ error_exit:
 		free(pmp, M_MSDOSFSMNT);
 		mp->mnt_data = NULL;
 	}
+	atomic_store_rel_ptr((uintptr_t *)&dev->si_mountpt, 0);
 	dev_rel(dev);
 	return (error);
 }
@@ -846,13 +845,10 @@ msdosfs_unmount(struct mount *mp, int mntflags)
 		BO_UNLOCK(bo);
 	}
 #endif
-	DROP_GIANT();
-	if (pmp->pm_devvp->v_type == VCHR && pmp->pm_devvp->v_rdev != NULL)
-		pmp->pm_devvp->v_rdev->si_mountpt = NULL;
 	g_topology_lock();
 	g_vfs_close(pmp->pm_cp);
 	g_topology_unlock();
-	PICKUP_GIANT();
+	atomic_store_rel_ptr((uintptr_t *)&pmp->pm_dev->si_mountpt, 0);
 	vrele(pmp->pm_devvp);
 	dev_rel(pmp->pm_dev);
 	free(pmp->pm_inusemap, M_MSDOSFSFAT);