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

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 62a84a96c6295044e24bad0d9383dbc2a50b128d..8a7995a7901106e6a97af50b844114a2d65f8874 100644 (file)
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -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;