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vfs: Add a function to lazily unmount all mounts from any dentry.

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The new function detach_mounts comes in two pieces.  The first piece
is a static inline test of d_mounpoint that returns immediately
without taking any locks if d_mounpoint is not set.  In the common
case when mountpoints are absent this allows the vfs to continue
running with it's same cacheline foot print.

The second piece of detach_mounts __detach_mounts actually does the
work and it assumes that a mountpoint is present so it is slow and
takes namespace_sem for write, and then locks the mount hash (aka
mount_lock) after a struct mountpoint has been found.

With those two locks held each entry on the list of mounts on a
mountpoint is selected and lazily unmounted until all of the mount
have been lazily unmounted.

v7: Wrote a proper change description and removed the changelog
    documenting deleted wrong turns.

Signed-off-by: Eric W. Biederman <[email protected]>
Signed-off-by: Al Viro <[email protected]>
This commit is contained in:
Eric W. Biederman 2013-10-03 01:31:18 -07:00 • committed by Al Viro
commit 80b5dce8c5
2 changed files with 40 additions and 0 deletions
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9
fs/mount.h
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@ -87,6 +87,15 @@ extern struct mount *__lookup_mnt_last(struct vfsmount *, struct dentry *);
extern bool legitimize_mnt(struct vfsmount *, unsigned);
extern void __detach_mounts(struct dentry *dentry);
static inline void detach_mounts(struct dentry *dentry)
{
if (!d_mountpoint(dentry))
return;
__detach_mounts(dentry);
}
static inline void get_mnt_ns(struct mnt_namespace *ns)
{
atomic_inc(&ns->count);